update-manager crashed with AttributeError in __init__(): module 'platform' has no attribute 'dist'

1. `$ lsb_release -rd` returns the following:

```
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04
```

==========

2. `$ apt-cache policy update-manager` returns the following:

```
update-manager:
  Installed: 1:19.04.8
  Candidate: 1:19.04.8
  Version table:
 *** 1:19.04.8 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu focal/main i386 Packages
        100 /var/lib/dpkg/status
```

==========

3. I expected `update-manager` to check for updates, an list any available package updates.
4. I think my `update-manager` crashed while it was getting package update headers.

==========

I had installed `python3.8` from Debian repo and used `update-alternatives` to set its priority level over `python3.7`.

```
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
```

`$ sudo update-alternatives --config python3` returns the following:

```
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.8 2 auto mode
  1 /usr/bin/python3.7 1 manual mode
  2 /usr/bin/python3.8 2 manual mode

Press <enter> to keep the current choice[*], or type selection number:
```

`$ journalctl | grep "update-manager"` returned the following:

```
Jan 09 00:46:14 nick-CM1630 sudo[11035]: nick : TTY=pts/0 ; PWD=/home/nick ; USER=root ; COMMAND=/usr/bin/update-manager --check-dist-upgrades --devel-release --proposed --debug
Jan 09 00:46:16 nick-CM1630 dbus-daemon[846]: [system] Activating service name='org.debian.apt' requested by ':1.2527' (uid=0 pid=11036 comm="/usr/bin/python3 /usr/bin/update-manager --check-d" label="unconfined") (using servicehelper)
Jan 09 00:46:28 nick-CM1630 update-notifier-crash[11409]: update-manager
Jan 09 00:51:00 nick-CM1630 whoopsie[30066]: [00:51:00] Parsing /var/crash/_usr_bin_update-manager.0.crash.
Jan 09 00:51:01 nick-CM1630 whoopsie[30066]: [00:51:00] Uploading /var/crash/_usr_bin_update-manager.0.crash.
Jan 09 00:51:03 nick-CM1630 sudo[13969]: root : TTY=unknown ; PWD=/root ; USER=nick ; ENV=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus ; COMMAND=/usr/bin/xdg-open https://bugs.launchpad.net/ubuntu/+source/update-manager/+filebug/66df57a6-32ac-11ea-a586-0025b3df357a?field.title=update-manager+crashed+with+AttributeError+in+__init__%28%29%3A+module+%27platform%27+has+no+attribute+%27dist%27
```

Debug log returned by `update-manager`:

```
DEBUG:AptClient.DebconfProxy:debconf socket: /tmp/aptdaemon-3uz3s6_d/debconf.socket
DEBUG:root:debconf.start()
DEBUG:root:debconf.stop()
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/UpdateManager/backend/InstallBackendAptdaemon.py", line 236, in _on_finished
    self._action_done(action,
  File "/usr/lib/python3/dist-packages/UpdateManager/backend/__init__.py", line 95, in _action_done
    self.window_main.start_available(is_cancelled_update)
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 233, in start_available
    self.refresh_cache()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 388, in refresh_cache
    self.update_list = UpdateList(self)
  File "/usr/lib/python3/dist-packages/UpdateManager/Core/UpdateList.py", line 211, in __init__
    self.dist = dist if dist else platform.dist()[2]
```

Found (3) files in `/var/crash`. I assume that log was "uploaded".

- /var/crash/_usr_bin_update-manager.0.crash
- /var/crash/_usr_bin_update-manager.0.upload
- /var/crash/_usr_bin_update-manager.0.uploaded

To solve my issue, I used `$ sudo update-alternatives --config python3` to give `python3.7` priority over `python3.8`:

```
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.8 2 auto mode
  1 /usr/bin/python3.7 1 manual mode
  2 /usr/bin/python3.8 2 manual mode

Press <enter> to keep the current choice[*], or type selection number: 1

update-alternatives: using /usr/bin/python3.7 to provide /usr/bin/python3 (python3) in manual mode
```

`$ sudo update-alternatives --config python3` returned:

```
There are 2 choices for the alternative python3 (providing /usr/bin/python3).

  Selection Path Priority Status
------------------------------------------------------------
  0 /usr/bin/python3.8 2 auto mode
* 1 /usr/bin/python3.7 1 manual mode
  2 /usr/bin/python3.8 2 manual mode

Press <enter> to keep the current choice[*], or type selection number:
```

Additional Actions

Since the "Software Updater" window froze and would not close, I killed the `update-notifier` and `gnome-software` processes using System Monitor. But that failed to close the frozen window. So I used the desktop's top pulldown menu to open another "Software Updater" which successfully opened and listed software updates. I closed the "Software Updater", and installed the updates using `$ sudo apt-get dist-upgrade -y`. Those packages were libmysofa0, libvncclient1, python3-httplib2 and usb-modeswitch, and they all installed successfully.

ProblemType: Crash
DistroRelease: Ubuntu 20.04
Package: update-manager 1:19.04.8
ProcVersionSignature: Ubuntu 5.0.0-37.40~18.04.1-generic 5.0.21
Uname: Linux 5.0.0-37-generic x86_64
ApportVersion: 2.20.11-0ubuntu15
Architecture: amd64
Date: Thu Jan 9 00:46:28 2020
ExecutablePath: /usr/bin/update-manager
InstallationDate: Installed on 2020-01-05 (3 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
InterpreterPath: /usr/bin/python3.8
PackageArchitecture: all
ProcCmdline: /usr/bin/python3 /usr/bin/update-manager --check-dist-upgrades --devel-release --proposed --debug
ProcEnviron:
 LANG=en_US.UTF-8
 TERM=xterm-256color
 PATH=(custom, no user)
 SHELL=/bin/bash
Python3Details: /usr/bin/python3.8, Python 3.8.0+, python3-minimal, 3.7.5-1ubuntu1
PythonArgs: ['/usr/bin/update-manager', '--check-dist-upgrades', '--devel-release', '--proposed', '--debug']
PythonDetails: N/A
SourcePackage: update-manager
Title: update-manager crashed with AttributeError in __init__(): module 'platform' has no attribute 'dist'
UpgradeStatus: Upgraded to focal on 2020-01-07 (1 days ago)
UserGroups: