Getting rid of another top-level file.
`coverage.py` has supported `pyproject.toml` since version 5.0, and
all versions of coverage so far work with python 2.7. We just need to
ensure that a version of coverage with the `toml` extra is installed
in the test environment.
I tested this with `coverage run`, `coverage report`, and `coverage html`.
This moves our `mypy` configuration from `.mypy.ini` to `.pyproject.toml`
and increases the minimum `mypy` version in the tests.
- [x] move `mypy` configuration to `pyproject.toml`
- [x] remove `.mypy.ini`
- [x] ensure that `mypy` version .900 or higher is used in tests