Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/keyword.pyi

@@ -0,0 +1,10 @@
+import sys
+from typing import Sequence, Text
+
+def iskeyword(s: Text) -> bool: ...
+
+kwlist: Sequence[str]
+
+if sys.version_info >= (3, 9):
+    def issoftkeyword(s: str) -> bool: ...
+    softkwlist: Sequence[str]