• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

• Add .pyi type stub files for the hightime module.
  • I started by using MonkeyType to generate type stubs based on a unit test run.
  • I manually updated the type stubs based on typeshed/stdlib/datetime.pyi.
  • I updated the __new__ type hints to reflect the expected args and kwargs, and added an overload for the "tzinfo in parameter 8" special case.
• Add mypy to tox.ini.
• Add type hints to the tests.
• Add build artifacts to .gitignore.

Why should this Pull Request be merged?

Fixes #4

What testing has been done?

Ran tox:

Success: no issues found in 7 source files
  py37-test: OK (1.41=setup[0.06]+cmd[0.06,0.09,1.19] seconds)
  py38-test: SKIP (0.03 seconds)
  py39-test: OK (1.34=setup[0.02]+cmd[0.05,0.09,1.19] seconds)
  pypy-test: SKIP (0.03 seconds)
  pypy3-test: SKIP (0.05 seconds)
  flake8: OK (0.84=setup[0.00]+cmd[0.05,0.09,0.70] seconds)
  black: OK (0.80=setup[0.00]+cmd[0.05,0.09,0.66] seconds)
  mypy: OK (1.09=setup[0.00]+cmd[0.05,0.09,0.95] seconds)
  congratulations :) (5.70 seconds)

Did some preliminary testing with projects that depend on hightime (measurement-plugin-python, nidaqmx-python) and found some typing bugs in nidaqmx-python that were hidden by hightime's lack of type hints.