Fix all invalid escape sequences caught by tests
Created originally on Bitbucket by jdufresne (Jon Dufresne)
Was already merged in Bitbucket before import, marked as merged by the import user
Starting Python 3.6, using a backslash that is not a valid escape sequence generates a deprecation warning. This will produce an error in a future Python version. For additional details, see the PYthon release notes:
A backslash-character pair that is not a valid escape sequence now generates a DeprecationWarning. Although this will eventually become a SyntaxError, that will not be for several Python releases.
Fixes warnings of the form
DeprecationWarning: invalid escape sequence .