add (overwrite with) mypy stubs, if available

stdlib/3/doctest.pyi

@@ -0,0 +1,9 @@
+# Stubs for doctest
+
+# NOTE: These are incomplete!
+
+from typing import Any, Tuple
+
+# TODO arguments missing
+def testmod(module: Any = None, *, name: str = None, globs: Any = None,
+            verbose: bool = None) -> Tuple[int, int]: ...