Migrate mypy config to pyproject.toml

[johnthagen]

Upstream tracking issues

• Support for pyproject.toml python/mypy#5205
• Add support to pyproject.toml python/mypy#5208

[johnthagen]

PR getting closer to approval: python/mypy#10219

[johnthagen]

Support has been merged into mypy master: python/mypy#10219 (review)