Support typing_extensions.overload in addition to typing.overload (#466)

They are synonyms and typing_extensions.overloads works on 3.5.1.

Author: msullivan

pyflakes/checker.py

@@ -622,7 +622,9 @@ def name_is_typing_overload(name):  # type: (str) -> bool
             if name in scope:
                 return (
                     isinstance(scope[name], ImportationFrom) and
-                    scope[name].fullName == 'typing.overload'
+                    scope[name].fullName in (
+                        'typing.overload', 'typing_extensions.overload',
+                    )
                 )
 
         return False

pyflakes/test/test_type_annotations.py

@@ -39,6 +39,23 @@ def g(s):
             return s
         """)
 
+    def test_typingExtensionsOverload(self):
+        """Allow intentional redefinitions via @typing_extensions.overload"""
+        self.flakes("""
+        from typing_extensions import overload
+
+        @overload
+        def f(s):  # type: (None) -> None
+            pass
+
+        @overload
+        def f(s):  # type: (int) -> int
+            pass
+
+        def f(s):
+            return s
+        """)
+
     def test_overload_with_multiple_decorators(self):
         self.flakes("""
             from typing import overload