Deprecation warning from importlib due to \dot, etc., in docstrings

See, e.g., 

  https://travis-ci.org/python-control/python-control/jobs/325460249#L2717

This particular one appears to be due to `\dot` at https://github.com/python-control/python-control/blob/7b2defe28034058eecdbde9207dc5854ab3faabd/control/statesp.py#L1056 ; the `\d` is interpreted as an (invalid) escape sequence.

The fix is probably to use a raw string.