Support Multidict[int] for pure-python version

[asvetlov]

__class_getitem__ is already provided by C Extension, making it work with the pure-extension too.

[codecov]

Codecov Report

Merging #678 (0072061) into master (0e0e98f) will decrease coverage by 0.12%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
- Coverage   93.77%   93.65%   -0.13%     
==========================================
  Files           5        5              
  Lines         498      504       +6     
==========================================
+ Hits          467      472       +5     
- Misses         31       32       +1     

Impacted Files	Coverage Δ
multidict/_multidict_py.py	99.12% <83.33%> (-0.29%)	⬇️
multidict/__init__.py	100.00% <100.00%> (ø)
multidict/_compat.py	100.00% <100.00%> (ø)

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