setup mypy type checking

[tswast]

Sub-tasks:

• Add a mypy session to the noxfile (possibly to the shared templates https://github.com/googleapis/synthtool/tree/master/synthtool/gcp/templates/python_library)
• Ensure our code passes type checks.
• Add type checking to github templates (possibly to the shared templates https://github.com/googleapis/synthtool/tree/master/synthtool/gcp/templates/python_library)
• Add type checks to required checks in sync-repo-settings
• Add py.typed file