Complete type annotations and PEP 561 compatibility

[mmaslowskicc]

What action do you want to perform

I want to use mypy with my project which uses pytest-postgresql in its test suite while not using the Any type.

What are the results

tests/conftest.py:8: error: Skipping analyzing 'pytest_postgresql': found
module but no type hints or library stubs  [import]
    from pytest_postgresql import factories
    ^
tests/conftest.py:8: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports

What are the expected results

pytest-postgresql has inline type annotations, mypy finds them (https://mypy.readthedocs.io/en/stable/installed_packages.html#installed-packages) and correctly checks the types.