This updates CVSClient to add formal support for dependency checks.

CVSClient.check_dependencies() now determines if cvs is installed.
CVSClient.get_local_path() now calls this instead of checking for

dependencies itself, emitting a warning if dependencies haven't already

been checked via setup() or has_dependencies().
Unit tests have been updated to include new dependency-related tests. If

cvs isn't installed, tests that depend on it will be automatically

skipped.

Unit tests pass on Python 3.7-3.11.