Don't treat functions with reversible dunder names as such

Previously, functions that had the same name as a reversible dunder
method (for example, __rpow__) were checked as if they were such a
method. This could lead to crashes and false positives.

Closes: python#5419

Author: srittau

mypy/checker.py

@@ -775,7 +775,7 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
                                            self.msg, context=fdef)
 
                 if name:  # Special method names
-                    if name in nodes.reverse_op_method_set:
+                    if defn.info and name in nodes.reverse_op_method_set:
                         self.check_reverse_op_method(item, typ, name, defn)
                     elif name in ('__getattr__', '__getattribute__'):
                         self.check_getattr_method(typ, defn, name)