Much of the RBTools codebase involves working with SCMClients. With a

lot being done to enhance SCMClients and prepare them for new diff

tools, sand with variations in expectations on parameters and types,

it's worth taking the time to really standardize the types in use and

the ways in which we define and invoke methods.
This change adds type annotations to all the function definitions and

class and instance attributes in BaseSCMClient, and to add

keyword-only arguments to many of the methods. This will enable type

checking to more strictly check any code going into RBTools from here on

out.
Many of the functions today return dictionaries, and we now define what

keys are expected in those by defining TypedDict-based types.
The subclasses haven't been cleaned up yet, but that can be handled

separately.

Unit tests pass on Python 3.7-3.11.