include current notification level in cache key

this prevents a cached regular expression being used that was created
with a different notification level.

For example, the following now generates the expected warning:

    In [1]: import re2
    In [2]: re2.compile('a*+')
    Out[2]: re.compile('a*+')
    In [3]: re2.set_fallback_notification(re2.FALLBACK_WARNING)
    In [4]: re2.compile('a*+')
    <ipython-input-5-041122e221c7>:1: UserWarning: WARNING: Using re module. Reason: bad repetition operator: *+
      re2.compile('a*+')
    Out[4]: re.compile('a*+')

src/compile.pxi

@@ -1,6 +1,6 @@
 
 def compile(pattern, int flags=0, int max_mem=8388608):
-    cachekey = (type(pattern), pattern, flags)
+    cachekey = (type(pattern), pattern, flags, current_notification)
     if cachekey in _cache:
         return _cache[cachekey]
     p = _compile(pattern, flags, max_mem)