Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/_markupbase.pyi

@@ -0,0 +1,8 @@
+from typing import Tuple
+
+class ParserBase:
+    def __init__(self) -> None: ...
+    def error(self, message: str) -> None: ...
+    def reset(self) -> None: ...
+    def getpos(self) -> Tuple[int, int]: ...
+    def unknown_decl(self, data: str) -> None: ...