fix: Python 3.7 typing imports

[cragwolfe]

Per issue #57 , fix typing imports in Python 3.7.

make test failed before these changes, and now works in a local Python-3.7 venv.

To be fair, getting all the deps installed in a Python-3.7 venv is a bit of pain, but I did that with pip install -e ., multiple calls to pip install -r requirements/___.in, then pip install any remaining missing packages.

Thanks to @firmai for providing a snippet to the fix.