refactor: declare TypeAliases explicitly - #652

OK if we target this for 4.0, since TypeAlias is not in Python 3.9?

if you are okay with it I would make a conditional import e.g. into _types.py: compat.py (or on every use, since compat seems unused):

try:
    from typing import TypeAlias
except ImportError:
     from typing_extensions import TypeAlias

@mistotebe I found a fallback strategy for Py3.9 - please have a look at the latest diff. All tests passing.