Enforce type checking using mypy to enable Python 3 migration

[zbeekman]

mypy allows python type checking to be enforced. This will help identify bugs, but, in particular, it will help the migration from Python2 to Python3.

See:

• https://twitter.com/grhmc/status/1233598648673538048
• https://github.com/asottile/pyupgrade
• https://mypy.readthedocs.io/en/latest/getting_started.html
• https://mypy.readthedocs.io/en/latest/existing_code.html#existing-code
• https://github.com/dropbox/pyannotate
• https://monkeytype.readthedocs.io/en/latest/index.html