Annotate and document kerning.py. (#813)

* Redefined variables for type security.

* Add type annotations.

* Remove unused imports.

* Add type annotations to methods in BaseKerning class

* Add and improve documentation and make other minor edits.

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Co-authored-by: knutnergaard <knutnergaard@users.noreply.github.com>
Co-authored-by: knutnergaard <15194233+knutnergaard@users.noreply.github.com>

Author: knutnergaard