This updates GitClient to add formal support for dependency checks.

GitClient.check_dependencies() now determines if git or git.cmd

(available on Windows) is installed.
GitClient.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(). The same is done

with the GitClient.git property, which used to be an attribute.
Unit tests have been updated to include new dependency-related tests. If

a suitable git tool isn't installed, tests that depend on it will be

automatically skipped.

Unit tests pass on Python 3.7-3.11.