bpo-32911: Revert bpo-29463. (GH-7121) (GH-7197)

Remove the docstring attribute of AST types and restore docstring
expression as a first stmt in their body.

Co-authored-by: INADA Naoki <methane@users.noreply.github.com>
diff --git a/Misc/NEWS.d/3.7.0a1.rst b/Misc/NEWS.d/3.7.0a1.rst
index 3962923..acecfcf 100644
--- a/Misc/NEWS.d/3.7.0a1.rst
+++ b/Misc/NEWS.d/3.7.0a1.rst
@@ -788,7 +788,7 @@
 Add ``docstring`` field to Module, ClassDef, FunctionDef, and
 AsyncFunctionDef ast nodes.  docstring is not first stmt in their body
 anymore.  It affects ``co_firstlineno`` and ``co_lnotab`` of code object for
-module and class.
+module and class. (Reverted in :issue:`32911`.)
 
 ..