xeger: avoid deprecated sre_parse module in python-3.11+

The undocumented sre_parse module got deprecated in Python 3.11 which leads
to build/test failures on Linux distributions making use of this version, e.g.
Ubuntu 23.04.

See bpo-47152: python/cpython#91308

rstr/xeger.py

@@ -1,5 +1,4 @@
 import random
-import sre_parse
 import string
 from itertools import chain
 import typing
@@ -10,6 +9,11 @@
 if typing.TYPE_CHECKING:
     from rstr.rstr_base import _Random
 
+try:
+    import re._parser as sre_parse
+except ImportError:  # Python < 3.11
+    import sre_parse  # type: ignore
+
 
 # The * and + characters in a regular expression
 # match up to any number of repeats in theory,