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: f66e47f98c ("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>

Documentation/sphinx/automarkup.py

@@ -15,6 +15,14 @@ else:
 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
@@ -22,22 +30,22 @@ from itertools import chain
 # :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