Revert "Filter out parameterized types. Fixes #2."

This reverts commit e34c923, no longer needed on supported Pythons.

singledispatch/__init__.py

@@ -117,7 +117,7 @@ def is_strict_base(typ):
     mro = []
     for typ in types:
         found = []
-        for sub in filter(_safe, typ.__subclasses__()):
+        for sub in typ.__subclasses__():
             if sub not in bases and issubclass(cls, sub):
                 found.append([s for s in sub.__mro__ if s in type_set])
         if not found:
@@ -132,13 +132,6 @@ def is_strict_base(typ):
     return _c3_mro(cls, abcs=mro)
 
 
-def _safe(class_):
-    """
-    Return if the class is safe for testing as subclass. Ref #2.
-    """
-    return not getattr(class_, '__origin__', None)
-
-
 def _find_impl(cls, registry):
     """Returns the best matching implementation from *registry* for type *cls*.