This updates SVNClient to add formal support for dependency checks.

SVNClient.check_dependencies() now determines if svn is installed.
SVNClient.get_local_path(), SVNClient.get_repository_info(), and

SVNClient.is_remote_only() now call this instead of checking for

dependencies themselves, emitting a warning if dependencies haven't

already been checked via setup() or has_dependencies().
Unit tests have been updated to test the dependency checks. With the new

auto-test skipping, individual tests will be skipped if svn isn't

installed, but tests that don't require svn will continue to run.

Unit tests pass on Python 3.7-3.11.