[Python-Dev] Make re.compile faster

[Victor Stinner]

> * RegexFlag.__and__ and __new__ is called very often.

Yeah, when the re module was modified to use enums for flags,
re.compile() became slower:
https://pyperformance.readthedocs.io/cpython_results_2017.html#slowdown

https://speed.python.org/timeline/#/?exe=12&ben=regex_compile&env=1&revs=200&equid=off&quarts=on&extr=on

It would be nice if internally we could use integers again to reduce
this overhead, without loosing the nice representation:

>>> re.I
<RegexFlag.IGNORECASE: 2>

Victor