add (overwrite with) mypy stubs, if available

stdlib/2.7/inspect.pyi

@@ -0,0 +1,11 @@
+# TODO incomplete
+from typing import Any, List, Tuple
+
+def isgenerator(object: Any) -> bool: ...
+
+class _Frame:
+    ...
+_FrameRecord = Tuple[_Frame, str, int, str, List[str], int]
+
+def currentframe() -> _FrameRecord: ...
+def stack(context: int = None) -> List[_FrameRecord]: ...