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[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/types.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC90eXBlcy5weQ==) | `92.30% <84.61%> (-7.70%)` | :arrow_down: | | [skll/learner/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9sZWFybmVyL3V0aWxzLnB5) | `93.31% <96.49%> (-0.42%)` | :arrow_down: | | [skll/learner/\_\_init\_\_.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9sZWFybmVyL19faW5pdF9fLnB5) | `97.18% <98.85%> (+0.03%)` | :arrow_up: | | [skll/experiments/\_\_init\_\_.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy9fX2luaXRfXy5weQ==) | `94.68% <100.00%> (+0.04%)` | :arrow_up: | | [skll/experiments/utils.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9leHBlcmltZW50cy91dGlscy5weQ==) | `93.38% <100.00%> (ø)` | | | [skll/learner/voting.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC9sZWFybmVyL3ZvdGluZy5weQ==) | `98.54% <100.00%> (+0.09%)` | :arrow_up: | | [skll/utils/constants.py](https://app.codecov.io/gh/EducationalTestingService/skll/pull/734?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC91dGlscy9jb25zdGFudHMucHk=) | `100.00% <100.00%> (ø)` | |

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