Add type hints to skll.learner module + other refactoring

[desilinguist]

• Add type hints to skll.learner
  • Modify skll.utils.constants to add type hint.
  • Refactor some learner code as needed.
• Add new test and tweak check
• Add new custom types.
• Add type annotations in some places to avoid circular dependencies.
• Fix type in documentation heading

This PR is part of #561.

[codecov]

Codecov Report

Patch coverage: 97.80% and project coverage change: -0.06 ⚠️

Comparison is base (f8c3da8) 95.26% compared to head (d407d26) 95.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #734      +/-   ##
==========================================
- Coverage   95.26%   95.20%   -0.06%     
==========================================
  Files          29       29              
  Lines        3461     3504      +43     
==========================================
+ Hits         3297     3336      +39     
- Misses        164      168       +4     

Impacted Files	Coverage Δ
skll/types.py	92.30% <84.61%> (-7.70%)	⬇️
skll/learner/utils.py	93.31% <96.49%> (-0.42%)	⬇️
skll/learner/__init__.py	97.18% <98.85%> (+0.03%)	⬆️
skll/experiments/__init__.py	94.68% <100.00%> (+0.04%)	⬆️
skll/experiments/utils.py	93.38% <100.00%> (ø)
skll/learner/voting.py	98.54% <100.00%> (+0.09%)	⬆️
skll/utils/constants.py	100.00% <100.00%> (ø)

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