docs: fix automarkup regression on Python 2

import re

from itertools import chain

#

# Python 2 lacks re.ASCII...

#

try:

    ascii_p3 = re.ASCII

except AttributeError:

    ascii_p3 = 0

#

# Regex nastiness.  Of course.

# Try to identify "function()" that's not already marked up some

# :c:func: block (i.e. ":c:func:`mmap()`s" flakes out), so the last

# bit tries to restrict matches to things that won't create trouble.

#

RE_function = re.compile(r'\b(([a-zA-Z_]\w+)\(\))', flags=re.ASCII)

RE_function = re.compile(r'\b(([a-zA-Z_]\w+)\(\))', flags=ascii_p3)

#

# Sphinx 2 uses the same :c:type role for struct, union, enum and typedef

#

RE_generic_type = re.compile(r'\b(struct|union|enum|typedef)\s+([a-zA-Z_]\w+)',

                             flags=re.ASCII)

                             flags=ascii_p3)

#

# Sphinx 3 uses a different C role for each one of struct, union, enum and

# typedef

#

RE_struct = re.compile(r'\b(struct)\s+([a-zA-Z_]\w+)', flags=re.ASCII)

RE_union = re.compile(r'\b(union)\s+([a-zA-Z_]\w+)', flags=re.ASCII)

RE_enum = re.compile(r'\b(enum)\s+([a-zA-Z_]\w+)', flags=re.ASCII)

RE_typedef = re.compile(r'\b(typedef)\s+([a-zA-Z_]\w+)', flags=re.ASCII)

RE_struct = re.compile(r'\b(struct)\s+([a-zA-Z_]\w+)', flags=ascii_p3)

RE_union = re.compile(r'\b(union)\s+([a-zA-Z_]\w+)', flags=ascii_p3)

RE_enum = re.compile(r'\b(enum)\s+([a-zA-Z_]\w+)', flags=ascii_p3)

RE_typedef = re.compile(r'\b(typedef)\s+([a-zA-Z_]\w+)', flags=ascii_p3)

#

# Detects a reference to a documentation page of the form Documentation/... with