Comment 1 for bug 1799926

May be related to python becoming stricter on backslash:

 https://bugs.python.org/issue27364

and from https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

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. (Contributed by Emanuel Barry in bpo-27364.)

can't see what version of pip is used or what version has changed for that matter.