Allow numbers in to_constant

Enums aren't effected if a int is passed along with the string.

Author: AdrielVelazquez

graphene/utils/str_converters.py

@@ -18,4 +18,4 @@ def to_snake_case(name):
 
 
 def to_const(string):
-    return re.sub('[\W|^(?=\d)]+', '_', string).upper()
+    return re.sub('[\W|^]+', '_', string).upper()