match: make the FLAG_RE pattern a raw string

PyCharm was complaining about invalid escape sequences since this was added
recently in 3eda36e9b3d6.

mercurial/match.py

@@ -1323,7 +1323,7 b' def _globre(pat):'
     return res
 
 
-FLAG_RE = util.re.compile(b'^\(\?([aiLmsux]+)\)(.*)')
+FLAG_RE = util.re.compile(br'^\(\?([aiLmsux]+)\)(.*)')
 
 
 def _regex(kind, pat, globsuffix):