Issue #28691: Fix warn_invalid_escape_sequence(): handle correctly
DeprecationWarning raised as an exception. First clear the current exception to
replace the DeprecationWarning exception with a SyntaxError exception.

Unit test written by Serhiy Storchaka.

Per suggestions by Ivan Levkivskyi.
Patch by Elvis Pranskevichus.

non-integer numbers.  Django tests depend on this.

non-integer numbers.  Django tests depend on this.

non-integer numbers.  Django tests depend on this.

non-integer numbers.  Django tests depend on this.

non-integer numbers.  Django tests depend on this.

"issue28082: use IntFlag for re constants" by Ethan Furman.

The re module is not more used in the site module and so adding "import enum"
to re.py doesn't impact python_startup benchmark anymore.

Patch by Matthieu S.

with a cosmetic change to add a commit message with the issue number,
missing from the previous two commits.