Add type hints to skll.data module & improve types in skll.config

[desilinguist]

1. Update pre-commit checks to include a check for commit message format and to exclude missing imports from the mypy check.
2. Add new custom types to skll.types module.
3. Add type hints to skll.data module
   • Refactor code to ensure correct types.
   • Add a lot of checks for labels, features, vectorizer being None that was previously unhandled.
   • Update docstrings to use the type hints.
4. Improve type hints in config/__init__.py
   • Use new ClassMap custom type.
   • Make sure pos_label is typed correctly.
   • Fix incorrect default value for sampler parameters.

[codecov]

Codecov Report

Patch coverage: 92.44% and project coverage change: -0.22 ⚠️

Comparison is base (d7f0eb2) 95.37% compared to head (482f4df) 95.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #730      +/-   ##
==========================================
- Coverage   95.37%   95.16%   -0.22%     
==========================================
  Files          29       29              
  Lines        3415     3451      +36     
==========================================
+ Hits         3257     3284      +27     
- Misses        158      167       +9     

Impacted Files	Coverage Δ
skll/data/featureset.py	90.00% <86.04%> (-0.57%)	⬇️
skll/data/writers.py	93.65% <93.93%> (-0.76%)	⬇️
skll/data/readers.py	90.36% <98.33%> (ø)
skll/config/__init__.py	94.75% <100.00%> (-0.02%)	⬇️
skll/types.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[desilinguist]

This is part of #561.