docs: fix automarkup regression on Python 2

It turns out that the Python 2 re module lacks the ASCII flag, so don't try
to use it there.

Fixes: f66e47f9 ("docs: automarkup.py: Fix regexes to solve sphinx 3 warnings")
Reported-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>