Use __init_subclass__ for subclass registration

[jparise]

Python 3.6 introduced __init_subclass__(cls), which provides a nicer subclass registration pattern than the previous metaclass-based approach.

[sigmavirus24]

If we're adding annotations should we start linting with mypy too? At least to avoid regressions

[sigmavirus24]

Personally I prefer import typing as t but won't inflict that on others

[jparise]

If we're adding annotations should we start linting with mypy too? At least to avoid regressions

Yes, it's on my "list" of things to try out. mypy reports a bunch of issues in the code's current state so there's some initial work to do before getting the CI job in place.