Introduction of detailed S3 parameters breaks existing AWS setups

Hi,

I just got an update of duplicity via snap to version 0.8.15.

Running my backup script running duplicity as follows:
```
duplicity incremental
--include-filelist backup_include.lst.txt \
--exclude-filelist backup_exclude.lst.txt \
--exclude-other-filesystems \
--exclude-device-files \
--progress \
--full-if-older-than 45D \
$HOME \
boto3+s3://<bucket-name>/<folder-name>
```

...resulted in the following error:
```
Traceback (innermost last):
  File "/snap/duplicity/111/bin/duplicity", line 104, in <module>
    with_tempdir(main)
  File "/snap/duplicity/111/bin/duplicity", line 90, in with_tempdir
    fn()
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/dup_main.py", line 1518, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/commandline.py", line 1200, in ProcessCommandLine
    backup, local_pathname = set_backend(args[0], args[1])
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/commandline.py", line 1071, in set_backend
    config.backend = backend.get_backend(bend)
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/backend.py", line 223, in get_backend
    obj = get_backend_object(url_string)
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/backend.py", line 209, in get_backend_object
    return factory(pu)
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/backends/s3_boto3_backend.py", line 85, in __init__
    self.reset_connection()
  File "/snap/duplicity/111/lib/python3.6/site-packages/duplicity/backends/s3_boto3_backend.py", line 93, in reset_connection
    self.s3 = boto3.resource(u's3', region_name=config.s3_region_name, endpoint_url=config.s3_endpoint_url)
 AttributeError: module 'duplicity.config' has no attribute 's3_region_name'
```

*Actual behaviour*
It seems like the changes introduced via https://gitlab.com/duplicity/duplicity/-/merge_requests/19 made the parameters `s3-region-name` and `s3-endpoint-url` mandatory, which breaks existing AWS S3 setups.

*Expected behaviour*
These parameters should be optional.

*Duplicity version*: 0.8.15
*Python version*: 3.6
*OS distro and version*: Ubuntu 20.04
*Target filesystem*: Linux to AWS S3

Thanks for your work!

EDIT: Added duplicity command.