py3: fix for Python 3.12 emitting SyntaxWarning on invalid escape sequences

Missed in 805d4a462abb:

  $ python3.12 mercurial/store.py
  mercurial/store.py:406: SyntaxWarning: invalid escape sequence '\.'
    EXCLUDED = re.compile(b'.*undo\.[^/]+\.(nd?|i)$')

mercurial/store.py

@@ -403,7 +403,7 b" REVLOG_FILES_VOLATILE_EXT = (b'.n', b'.n"
 # some exception to the above matching
 #
 # XXX This is currently not in use because of issue6542
-EXCLUDED = re.compile(b'.*undo\.[^/]+\.(nd?|i)$')
+EXCLUDED = re.compile(br'.*undo\.[^/]+\.(nd?|i)$')
 
 
 def is_revlog(f, kind, st):