Provisional support for PEP484 Type Hint Annotations

New public API should be type hinted in ``.pyi`` stub files (except ``pyplot`` and tests

which are typed in-line).

Tests should be type hinted minimally, essentially only when ``mypy`` generates errors.

CI and configuration for running ``mypy`` have been added.

Generation of ``pyplot.py`` requires ``black``

The autogenerated portions of ``pyplot.py`` use ``black`` autoformatting to ensure

syntax-correct, readable output code.

As such ``black`` is now a development and test requirement (for the test which

regenerates ``pyplot``).