Cache .mypy_cache on CI

[cgarciae]

What does this PR do?

Caches the .mypy_cache folder weekly using the Cache action to speedup mypy tests on CI.

[codecov-commenter]

Codecov Report

Merging #2819 (c32e04a) into main (a309273) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2819      +/-   ##
==========================================
- Coverage   81.26%   81.25%   -0.01%     
==========================================
  Files          53       53              
  Lines        5680     5677       -3     
==========================================
- Hits         4616     4613       -3     
  Misses       1064     1064              

Impacted Files	Coverage Δ
flax/linen/recurrent.py	100.00% <ø> (ø)
flax/linen/module.py	91.96% <0.00%> (-0.15%)	⬇️
flax/errors.py	85.45% <0.00%> (-0.14%)	⬇️
flax/core/scope.py	90.10% <0.00%> (-0.03%)	⬇️
flax/training/prefetch_iterator.py	0.00% <0.00%> (ø)

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