Fix mypy errors

[foarsitter]

The github actions fail because of some inconvenient mypy cases, this pull request address them.

[codecov]

Codecov Report

Merging #626 (e611a62) into master (2e40dc6) will decrease coverage by 26.0%.
The diff coverage is 66.7%.

@@            Coverage Diff            @@
##           master    #626      +/-   ##
=========================================
- Coverage    83.3%   57.4%   -25.9%     
=========================================
  Files          21      39      +18     
  Lines        1045    2515    +1470     
  Branches        0      74      +74     
=========================================
+ Hits          870    1442     +572     
- Misses        175    1057     +882     
- Partials        0      16      +16     

Flag	Coverage Δ
mypy	33.6% <66.7%> (?)
tests	83.3% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_backend.py	14.4% <0.0%> (ø)
tests/conftest.py	100.0% <100.0%> (ø)
django_redis/pool.py	77.0% <0.0%> (-13.4%)	⬇️
django_redis/client/sharded.py	86.1% <0.0%> (-6.6%)	⬇️
django_redis/client/sentinel.py	77.3% <0.0%> (-3.6%)	⬇️
django_redis/client/herd.py	23.4% <0.0%> (-2.4%)	⬇️
django_redis/cache.py	97.0% <0.0%> (-2.2%)	⬇️
django_redis/serializers/pickle.py	100.0% <0.0%> (ø)
tests/settings/sqlite_sentinel.py	100.0% <0.0%> (ø)
tests/settings/sqlite.py	100.0% <0.0%> (ø)
... and 23 more

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[WisdomPill]

thanks for the contribution!

[foarsitter]

Thank you for your quick response and maintaining this package!