Use collections.abc where applicable

[musicinmybrain]

Beginning with Python 3.3 (the earliest currently-supported version)
Sequence, Mapping, MutableMapping, and the other abstract base classes
for containers appear in in collections.abc. Beginning with Python 3.10,
they are removed from collections.

Using the collections.abc import where applicable provides
forward-compatibility with Python 3.10+ without breaking Python 3.3+.

(Some additional changes are still needed for Python 3.7–3.10 compatibility.)

[musicinmybrain]

The last issue I see for enabling Python 3.7–3.10 support is that GenericMeta and CallableMeta have been removed from typing in the standard library. I’m not sure at a glance what the correct workaround is.