Add type annotations

[ajnelson-nist]

With these annotations, the following command (with the package types-pytz also installed) reports no issues:

mypy case_mapping example.py tests

Adding type review to unit testing is left for a future patch series.

References:

• https://docs.python.org/3/library/typing.html#the-type-of-class-objects

Disclaimer: Participation by NIST in the creation of the documentation of mentioned software is not intended to imply a recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that any specific software is necessarily the best available for the purpose.

[ajnelson-nist]

I believe this does not impact #38 or #39.

[ajnelson-nist]

This PR is ready for review and merge.

[ajnelson-nist]

Adding type review to CI is in #41 . Merging that PR would merge this PR.