fixes for Django 5.1, add support for new STORAGES setting

[domvie]

Description

• Add custom get_storage_class to replace the removed method in Django 5.1 and avoid breaking changes
• Add support for new STORAGES setting using the 'dbbackup' alias.

Example:

STORAGES = {
    "default": {
        "BACKEND": "django.core.files.storage.FileSystemStorage",
        "OPTIONS": ...,
    },
    "dbbackup": {
        "BACKEND": "django.core.files.storage.FileSystemStorage",
        "OPTIONS": {
            "location": "/my/backup/dir/",
        },
    },
}

The previous DBBACKUP_STORAGE and DBBACKUP_STORAGE_OPTIONS are still valid and take precedence.

Fixes #522 and should also fix #523.

Please double check if I missed anything.

What confused me a bit was that for the DB Storage this package always uses the configured DBBACKUP_STORAGE, while the mediabackup storage always uses the default django storage. I kept this behavior to not break anything.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.97%. Comparing base (8f1307e) to head (ac00a03).
Report is 2 commits behind head on master.

Files	Patch %	Lines
dbbackup/settings.py	62.50%	1 Missing and 2 partials ⚠️
dbbackup/storage.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   91.40%   90.97%   -0.44%     
==========================================
  Files          19       19              
  Lines         873      886      +13     
  Branches      157      159       +2     
==========================================
+ Hits          798      806       +8     
- Misses         40       43       +3     
- Partials       35       37       +2     

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[Archmonger]

Converting to draft until tests are added.

[Archmonger]

I'm okay with a breaking change that completely removes the DBBACKUP_STORAGE setting, especially given the mismatch with mediabackup.
Would you like to do that within this PR? Otherwise, it'll be handled in a follow up PR.

[domvie]

I think I'd prefer that as another PR. That would also imply dropping support for Django <4.2 and would require a rewrite of some existing tests (e.g. I believe FakeStorage does currently not work for mediabackup etc.)

[rapsli]

great. Just ran into this. When will this be released on pip?

[Archmonger]

Will be released today.