DeprecationWarning will continue to be emitted for invalid escape
sequences in string and bytes literals just as it did in 3.7.
SyntaxWarning may be emitted in the future. But per mailing list
discussion, we don't yet know when because we haven't settled on how to
do so in a non-disruptive manner.
(Applies 4c5b6bac24 to the master branch).
(This is https://github.com/python/cpython/pull/15142 for master/3.9)
https://bugs.python.org/issue32912
Automerge-Triggered-By: @gpshead
Gregory P. Smith