Escaping warning in usb2can serial selector

[hardbyte]

Noticed a warning in the travis-ci output - https://travis-ci.org/hardbyte/python-can/jobs/531296370

test/test_detect_available_configs.py::TestDetectAvailableConfigs::test_count_returned
  /home/travis/build/hardbyte/python-can/can/interfaces/usb2can/serial_selector.py:42: DeprecationWarning: invalid escape sequence \c
    objSWbemServices = objWMIService.ConnectServer(".", "root\cimv2")
-- Docs: https://docs.pytest.org/en/latest/warnings.html

[felixdivo]

This is why I created #570. ;)