Add __future__.generator_stop for Python 3.5

Author: graingert

stdlib/3.5/__future__.pyi

@@ -0,0 +1,14 @@
+from sys import _version_info
+
+class _Feature:
+    def getOptionalRelease(self) -> _version_info: ...
+    def getMandatoryRelease(self) -> _version_info: ...
+
+absolute_import = ...  # type: _Feature
+division = ...  # type: _Feature
+generators = ...  # type: _Feature
+nested_scopes = ...  # type: _Feature
+print_function = ...  # type: _Feature
+unicode_literals = ...  # type: _Feature
+with_statement = ...  # type: _Feature
+generator_stop = ...  # type: _Feature