Distribute types

[cjolowicz]

Supply a py.typed marker file in the package directory, as described in PEP 561.

See also https://mypy.readthedocs.io/en/stable/installed_packages.html

[cjolowicz]

This should also allow us to revert the addition of mypy_path, provided we also install the package into the Nox session for mypy.

Set mypy_path in the configuration file pointing it to the src directory.
This is required to avoid errors about missing imports related to your package
when mypy is invoked on files located outside of the package, such as modules in
the test suite. For example, pre-commit runs mypy on staged files only, so this
would be a common issue with our mypy pre-commit hook.