Start getting mypy --strict passing

[jacobtomlinson]

Taking first steps towards #428.

I started this PR by adding some type annotations to tests before deciding to just exclude tests, but I don't see ay harm in leaving those changes in.

I think the only thing we really care about is having good type annotations on the kr8s module itself, so I've excluded all other Python code from mypy linting.

Other typing fixes here include using __all__ to explicitly re-export things at the kr8s and kr8s.asyncio levels and adding annotations to the exceptions submodule.

After this PR there are 237 errors when running mypy --strict ..

[codecov]

Codecov Report

Attention: Patch coverage is 93.47826% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.28%. Comparing base (87063fc) to head (827391b).
Report is 119 commits behind head on main.

Files with missing lines	Patch %	Lines
kr8s/_objects.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
+ Coverage   94.61%   95.28%   +0.66%     
==========================================
  Files          29       30       +1     
  Lines        3141     3666     +525     
==========================================
+ Hits         2972     3493     +521     
- Misses        169      173       +4     

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