This introduces rbtools.clients.base.registry.SCMClientRegistry, which

keeps track of all the available BaseSCMClient subclasses. This

enables fetching a client by ID, iterating through all clients, and

registering new ones.
The registry is accessible via a rbtools.clients.scmclient_registry

instance.
SCMClient classes should now set a scmclient_id attribute. This will

be mandated in RBTools 5. For now, any loaded via entry point that lack

an ID will have one assigned, with a warning.
To improve performance and to avoid packaging-related complications,

the default list of clients are now fully inline, rather than

introspected via entrypoints. This matches what we've been doing within

Review Board. The registry still scans entrypoints for third-party

packages, but only when needed (if listing all clients or if a lookup

fails to find it in the built-in list).
Entry points use the modern importlib.metadata.entry_points API. This

has only solidified as of Python 3.10 (3.8/3.9 had it but it's not

compatible), so we pull in the official backport for those versions.

Unit tests pass.
Posted this change for review.
Ran mypy and pyright on the new registry code, with no errors

or warnings.