Add Django 4.1 to test matrix

[lorddaedra]

Added support for Django 4.1

[codecov-commenter]

Codecov Report

Base: 88.77% // Head: 85.43% // Decreases project coverage by -3.33% :warning:

Coverage data is based on head (c821a36) compared to base (66ca631). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #248 +/- ## ========================================== - Coverage 88.77% 85.43% -3.34% ========================================== Files 6 4 -2 Lines 490 364 -126 Branches 87 48 -39 ========================================== - Hits 435 311 -124 + Misses 40 39 -1 + Partials 15 14 -1 ``` | [Impacted Files](https://codecov.io/gh/AndrewIngram/django-extra-views/pull/248?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [extra\_views/formsets.py](https://codecov.io/gh/AndrewIngram/django-extra-views/pull/248/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZXh0cmFfdmlld3MvZm9ybXNldHMucHk=) | | | | [extra\_views/\_\_init\_\_.py](https://codecov.io/gh/AndrewIngram/django-extra-views/pull/248/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZXh0cmFfdmlld3MvX19pbml0X18ucHk=) | | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[jonashaag]

Thanks a lot!