fix nice_identifier regex

Cython/Compiler/Code.py

@@ -289,7 +289,7 @@ class PyObjectConst(object):
 
 possible_unicode_identifier = re.compile(ur"(?![0-9])\w+$", re.U).match
 possible_bytes_identifier = re.compile(r"(?![0-9])\w+$".encode('ASCII')).match
-nice_identifier = re.compile('^[a-zA-Z0-0_]+$').match
+nice_identifier = re.compile('^[a-zA-Z0-9_]+$').match
 
 class StringConst(object):
     """Global info about a C string constant held by GlobalState.