Python 3.9 Rearranging typing and collections.abc

This is glorious.

There was this sort of awkward shoving match between typing and collections.abc. Both had generic type definitions and it was -- often -- unclear how to use them.

See PEP 585. Now they are all unified into a much happier family.

And. We wind up writing things like this.

import collections.abc
import typing
import sys

if sys.version_info >= (3, 9):
    BucketCollection = collections.abc.Collection[Sample]
else:
    BucketCollection = typing.Collection[Sample]

Now we can have code that passes tests for 3.8 and 3.9. And at some point we can cut off the 3.8 support and delete the ancient alternative definition.

I'm delighted to be able to move forward with a much simpler future in which collections are in the collections.abc and other, more foundational stuff is in typing.