Add unittest.result.failfast. (#3596)

stdlib/3/unittest/__init__.pyi

@@ -6,7 +6,7 @@ from types import ModuleType
 from unittest.async_case import *
 from unittest.case import *
 from unittest.loader import *
-from unittest.result import *
+from unittest.result import TestResult as TestResult
 from unittest.runner import *
 from unittest.signals import *
 from unittest.suite import *

stdlib/3/unittest/result.pyi

@@ -1,4 +1,4 @@
-from typing import List, Optional, Tuple, Type, Union
+from typing import Any, Callable, List, Optional, Tuple, Type, TypeVar, Union
 from types import TracebackType
 import unittest.case
 
@@ -8,6 +8,11 @@ _SysExcInfoType = Union[
     Tuple[None, None, None],
 ]
 
+_F = TypeVar("_F", bound=Callable[..., Any])
+
+# undocumented
+def failfast(method: _F) -> _F: ...
+
 
 class TestResult:
     errors: List[Tuple[unittest.case.TestCase, str]]