The package from rocky-proposed worked for me. Version info below:
python3-glance-store:
  Installed: 0.26.1-0ubuntu2.1~cloud0
  Candidate: 0.26.1-0ubuntu2.1~cloud0
  Version table:
 *** 0.26.1-0ubuntu2.1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-proposed/rocky/main amd64 Packages
        100 /var/lib/dpkg/status
     0.26.1-0ubuntu2~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
     0.23.0-0ubuntu1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

Test output:

$ openstack image create --public --file /home/ubuntu/images/bionic-server-cloudimg-amd64.img bionic-test
500 Internal Server Error: The server has either erred or is incapable of performing the requested operation. (HTTP 500)
                                  
$ juju run --unit glance/0 "apt-cache policy python3-glance-store"
python3-glance-store:
  Installed: 0.26.1-0ubuntu2~cloud0
  Candidate: 0.26.1-0ubuntu2~cloud0
  Version table:
 *** 0.26.1-0ubuntu2~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
        100 /var/lib/dpkg/status
     0.23.0-0ubuntu1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages



$ juju run --unit glance/0 "add-apt-repository cloud-archive:rocky-proposed --yes --update"                                                               
...
$ juju run --unit glance/0 "apt install --yes python3-glance-store; systemctl restart glance-api"
...
(clients) ubuntu@gnuoy-bastion2:~/branches/nova-compute$ juju run --unit glance/0 "apt-cache policy python3-glance-store"
python3-glance-store:
  Installed: 0.26.1-0ubuntu2.1~cloud0
  Candidate: 0.26.1-0ubuntu2.1~cloud0
  Version table:
 *** 0.26.1-0ubuntu2.1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-proposed/rocky/main amd64 Packages
        100 /var/lib/dpkg/status
     0.26.1-0ubuntu2~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
     0.23.0-0ubuntu1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

$ openstack image create --public --file /home/ubuntu/images/bionic-server-cloudimg-amd64.img bionic-test
+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Field            | Value                                                                                                                                                                                      |
+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| checksum         | c8994590c7d61dc68922e461686ef936                                                                                                                                                           |
| container_format | bare                                                                                                                                                                                       |
| created_at       | 2019-05-22T07:41:28Z                                                                                                                                                                       |
| disk_format      | raw                                                                                                                                                                                        |
| file             | /v2/images/788db968-ea48-4b4f-8c91-4e15d23dbe4c/file                                                                                                                                       |
| id               | 788db968-ea48-4b4f-8c91-4e15d23dbe4c                                                                                                                                                       |
| min_disk         | 0                                                                                                                                                                                          |
| min_ram          | 0                                                                                                                                                                                          |
| name             | bionic-test                                                                                                                                                                                |
| owner            | 3d4ca9d5799546bd852db00ee6d5d4c0                                                                                                                                                           |
| properties       | os_hash_algo='sha512', os_hash_value='be4993640deb7eb99b07667213b1fe3a9145df2c0ed5c72cf786a621fe64e93fb543cbb3fafa9a130988b684da432d2a55493c50e77a9dfe336e7ed996be92d9', os_hidden='False' |
| protected        | False                                                                                                                                                                                      |
| schema           | /v2/schemas/image                                                                                                                                                                          |
| size             | 348127232                                                                                                                                                                                  |
| status           | active                                                                                                                                                                                     |
| tags             |                                                                                                                                                                                            |
| updated_at       | 2019-05-22T07:41:46Z                                                                                                                                                                       |
| virtual_size     | None                                                                                                                                                                                       |
| visibility       | public                                                                                                                                                                                     |
+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+