Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/asyncio/compat.pyi

@@ -0,0 +1,8 @@
+import sys
+from typing import List
+
+if sys.version_info < (3, 7):
+    PY34: bool
+    PY35: bool
+    PY352: bool
+    def flatten_list_bytes(list_of_data: List[bytes]) -> bytes: ...