Make unittest.removeHandler() an overloaded function.

See https://docs.python.org/3/library/unittest.html#unittest.removeHandler

- Without args it removes the signal handler.
- With a function arg it is a test decorator that temporarily removes
  the handler while the test is running.

stdlib/3/unittest/__init__.pyi

@@ -327,4 +327,7 @@ def main(module: str = ..., defaultTest: _DefaultTestType = ...,
 def installHandler() -> None: ...
 def registerResult(result: TestResult) -> None: ...
 def removeResult(result: TestResult) -> None: ...
-def removeHandler(function: Optional[_FT]) -> _FT: ...
+@overload
+def removeHandler() -> None: ...
+@overload
+def removeHandler(function: _FT) -> _FT: ...