Followup to 90fc8980bb.
<!--
Thanks for your contribution!
Please read this comment in its entirety. It's quite important.
# Pull Request title
It should be in the following format:
```
bpo-NNNN: Summary of the changes made
```
Where: bpo-NNNN refers to the issue number in the https://bugs.python.org.
Most PRs will require an issue number. Trivial changes, like fixing a typo, do not need an issue.
# Backport Pull Request title
If this is a backport PR (PR made against branches other than `master`),
please ensure that the PR title is in the following format:
```
[X.Y] <title from the original PR> (GH-NNNN)
```
Where: [X.Y] is the branch name, e.g. [3.6].
GH-NNNN refers to the PR number from `master`.
-->
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>
* Add Py_UNREACHABLE() as an alias to abort().
* Use Py_UNREACHABLE() instead of assert(0)
* Convert more unreachable code to use Py_UNREACHABLE()
* Document Py_UNREACHABLE() and a few other macros.
'invalid character in identifier' now is raised instead of
'f-string: empty expression not allowed' if a subexpression contains
only whitespaces and they are not accepted by Python parser.
* bpo-29463: Add docstring field to some AST nodes.
ClassDef, ModuleDef, FunctionDef, and AsyncFunctionDef has docstring
field for now. It was first statement of there body.
* fix document. thanks travis!
* doc fixes
