This fleshes out the type hints in SCMToolRegistry, ensuring that the

registration implementation code and consumers of the registry are all

aware of what types are being put in and returned.
Some code has been modernized in the process. Namely, converting

super() calls to their short form, and leveraging yield from and

f-strings.
This is in preparation for some work that will lead to new SCM support.

Unit tests pass.
Type checkers pass.
Verified the typing when fetching resulting tools.