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'