ci: add mypy type checking and fix/exclude minor issues

[gruebel]

This PR

• most of the code has type annotation, so this adds a type checking task inside pre-commit, which will be triggered during CI
• had to adjust the StrEnum import to use the Python system info, so the type checker can properly handle it
• the ignores in MappingProxyType are needed, because the original type hints are different

Follow-up Tasks

• plan is to enable more and more mypy checks with follow-up PRs till strict mode can be enabled

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a853b85) 93.82% compared to head (4497d2c) 93.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
+ Coverage   93.82%   93.83%   +0.01%     
==========================================
  Files          16       16              
  Lines         437      438       +1     
==========================================
+ Hits          410      411       +1     
  Misses         27       27              

Flag	Coverage Δ
unittests	93.83% <100.00%> (+0.01%)	⬆️

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[federicobond]

There is no openfeature/__init__.py file to turn openfeature into an implicit namespace package. See #199

Other than that, LGTM.

[gruebel]

ok, no problem. Need to adjust the mypy config a bit to make it work.