[$] Escape sequences in Python strings

A change for Python 3.8-currently in beta-has produced someuser-visible warnings, but the problem is often in code that a user cannot(or should not) change: third-party modules. The problem that the warningis trying to highlight is real, however. The upshot is that the handling ofescape sequences (or non escape sequences, in truth) inPython string literals is in a rather messy state at this point.