test django 4.1

[jerch]

Fixes #117.

[jerch]

Oh well, this is abit more involved due to the changes from https://github.com/django/django/pull/15318

Needed changes:

• fix all test cases to exit early if pk=None
• document the changed behavior of backward managers behaving now similar to M2M relations

[coveralls]

Pull Request Test Coverage Report for Build 2790650087

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.957%

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Totals
Change from base Build 2183942293:	0.0%
Covered Lines:	1109
Relevant Lines:	1153

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💛 - Coveralls