Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/unittest/signals.pyi

@@ -0,0 +1,12 @@
+import unittest.result
+from typing import Any, Callable, TypeVar, overload
+
+_F = TypeVar("_F", bound=Callable[..., Any])
+
+def installHandler() -> None: ...
+def registerResult(result: unittest.result.TestResult) -> None: ...
+def removeResult(result: unittest.result.TestResult) -> bool: ...
+@overload
+def removeHandler(method: None = ...) -> None: ...
+@overload
+def removeHandler(method: _F) -> _F: ...