Add __future__.generator_stop for Python 3.5

Author: Thomas Grainger

stdlib/3/__future__.pyi

@@ -1,8 +1,8 @@
-from sys import _version_info
+import sys
 
 class _Feature:
-    def getOptionalRelease(self) -> _version_info: ...
-    def getMandatoryRelease(self) -> _version_info: ...
+    def getOptionalRelease(self) -> sys._version_info: ...
+    def getMandatoryRelease(self) -> sys._version_info: ...
 
 absolute_import = ...  # type: _Feature
 division = ...  # type: _Feature
@@ -11,3 +11,6 @@ nested_scopes = ...  # type: _Feature
 print_function = ...  # type: _Feature
 unicode_literals = ...  # type: _Feature
 with_statement = ...  # type: _Feature
+
+if sys.version_info[:2] >= (3, 5):
+    generator_stop = ...  # type: _Feature