Fix mypy checks

[browniebroke]

I think that the mypy tests are broken (See #183), so fixing them here before.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6b24fae) 98.23% compared to head (dcdd232) 98.23%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files           3        3           
  Lines         227      227           
=======================================
  Hits          223      223           
  Misses          4        4           

Flag	Coverage Δ
unittests	98.23% <ø> (ø)

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[browniebroke]

Looks like the failures are for older version of Django (3.1):

py311-dj31-drf314-mypy: commands[0]> mypy example
  /home/runner/work/drf-writable-nested/drf-writable-nested/.tox/py311-dj31-drf314-mypy/lib/python3.11/site-packages/django/utils/encoding.py:266: DeprecationWarning: 'locale.getdefaultlocale' is deprecated and slated for removal in Python 3.15. Use setlocale(), getencoding() and getlocale() instead.
    encoding = locale.getdefaultlocale()[1] or 'ascii'
  /home/runner/work/drf-writable-nested/drf-writable-nested/.tox/py311-dj31-drf314-mypy/lib/python3.11/site-packages/django-stubs/contrib/sites/models.pyi:13: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
  https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
  If this issue continues with mypy master, please report a bug at [https://github.com/python/mypy/issues](https://github.com/python/mypy/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc)
  version: 1.7.1
  /home/runner/work/drf-writable-nested/drf-writable-nested/.tox/py311-dj31-drf314-mypy/lib/python3.11/site-packages/django-stubs/contrib/sites/models.pyi:13: : note: please use --show-traceback to print a traceback when reporting a bug
  py311-dj31-drf314-mypy: exit 2 (1.35 seconds) /home/runner/work/drf-writable-nested/drf-writable-nested> mypy example pid=2330
py311-dj31-drf314-mypy: FAIL ✖ in 8.22 seconds

The latest versions of django-stubs and djangorestframework-stubs are for a specific version of Django and DRF, so I feel like we shouldn't type check for older versions... Thoughts?

[browniebroke]

Alternatively, drop support for Django <3.2, which has been long unsupported:

[browniebroke]

Tests are now passing, but it fails to upload to codecov... It seems unrelated to my change, but if someone can help resolving this, that would be appreciated. Thanks