This updates the TFS wrapper classes to switch over from execute() to

run_process(). This gives us greater type safety across TFS, which is

important in keeping this client implementation maintainable.
All call sites now explicitly read either the byte stream or the Unicode

stream, depending on their needs. There's no longer a risk of expecting

one type and ending up with another.
Unit tests have been updated to spy on run_process_exec(), which gives

us a safe, checked way of returning data for tests that guarantees

proper interpreration by TFS.

Unit tests pass on Python 3.7-3.11.