Python: Understand multiple parse mode flags specified in a regular expression string

[geoffw0]

Understand / correctly parse multiple parse mode flags specified in a regular expression string, for example i and s in:

(?is)abc.*

Previously we were only recognizing the first flag (i).

This is a backport of part of the Swift PR #13715 .

[yoff]

Very nice PR, good choice of tests. Also, thanks for doing this 🙏

[yoff]

The test failures can be explained by flag_group_start now only matching (? and not also the first mode character.
They do pose the question if mode characters should not be considered normal characters. They do not seem like good candidates for first and last items to be matched...