drop support for python 3.6

[jpinner-lyft]

• mypy >= 0.800 is required for python 3.9 and >= 0.930 is required for pypy (Allow mypy to run self check on PyPy python/mypy#11350)
• GenericMeta is no longer necessary on python 3.7+ (Impossible to specify a metaclass for a Generic class python/typing#449)
• disabling mypy tests on pypy even though they pass because they take > 5 minutes to run