Python 3.7 removes an isinstance(node.body[0], Expr) check ast.get_docstring() that makes it crash when you pass in AST nodes of modules or functions that do not have docstrings.

Steps to reproduce:

- git clone https://github.com/mgedmin/findimports
- cd findimports
- tox -e py37

The failure looks like this:

    Traceback (most recent call last):
      ...
      File "/home/mg/src/findimports/findimports.py", line 337, in find_imports_and_track_names
        visitor.visit(root)
      File "/home/mg/opt/python37/lib/python3.7/ast.py", line 262, in visit
        return visitor(node)
      File "/home/mg/src/findimports/findimports.py", line 169, in visitSomethingWithADocstring
        self.processDocstring(ast.get_docstring(node, clean=False), lineno)
      File "/home/mg/opt/python37/lib/python3.7/ast.py", line 211, in get_docstring
        node = node.body[0].value
    AttributeError: 'Import' object has no attribute 'value'

I think this fix is worth cherry-picking in 3.7.0.

New changeset 08f127a3cad8ce4eb281d30d9488c91b0fd7cfed by Serhiy Storchaka in branch 'master':
bpo-33851: Fix ast.get_docstring() for a node that lacks a docstring. (GH-7682)
https://github.com/python/cpython/commit/08f127a3cad8ce4eb281d30d9488c91b0fd7cfed

New changeset a50b825c18a92655f3dd7939e793fa3d4440d886 by Miss Islington (bot) in branch '3.7':
bpo-33851: Fix ast.get_docstring() for a node that lacks a docstring. (GH-7682)
https://github.com/python/cpython/commit/a50b825c18a92655f3dd7939e793fa3d4440d886

@mgedmin, Just to be certain, are you able to verify that the change in PR 7701 fixes the problem you see with 3.7.0rc1?

I've ran my findimports testsuite with CPython's git commit a50b825c18a92655f3dd7939e793fa3d4440d886 and my tests passed, so yes.

New changeset 235d0fb93cb456701ce24fad2bc307e9f0312d73 by Ned Deily (Miss Islington (bot)) in branch '3.7':
bpo-33851: Fix ast.get_docstring() for a node that lacks a docstring. (GH-7682)
https://github.com/python/cpython/commit/235d0fb93cb456701ce24fad2bc307e9f0312d73

Fixed in 3.7.0.  Thanks, everyone!