pylint does not work with Python 3.7

In cosmic, Python 3.7 was added, but pylint does not work with it. Test case:

```
$ echo "print('test')" > test.py
$ # python3.7 -m pylint test.py
No config file found, using default configuration
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/astroid/decorators.py", line 89, in wrapped
    res = next(generator)
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/astroid/decorators.py", line 104, in wrapped
    raise StopIteration
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3/dist-packages/pylint/__main__.py", line 7, in <module>
    pylint.run_pylint()
  File "/usr/lib/python3/dist-packages/pylint/__init__.py", line 16, in run_pylint
    Run(sys.argv[1:])
  File "/usr/lib/python3/dist-packages/pylint/lint.py", line 1347, in __init__
    linter.check(args)
  File "/usr/lib/python3/dist-packages/pylint/lint.py", line 768, in check
    self._do_check(files_or_modules)
  File "/usr/lib/python3/dist-packages/pylint/lint.py", line 901, in _do_check
    self.check_astroid_module(ast_node, walker, rawcheckers, tokencheckers)
  File "/usr/lib/python3/dist-packages/pylint/lint.py", line 980, in check_astroid_module
    walker.walk(ast_node)
  File "/usr/lib/python3/dist-packages/pylint/utils.py", line 1011, in walk
    cb(astroid)
  File "/usr/lib/python3/dist-packages/pylint/checkers/base.py", line 1571, in visit_module
    self._check_docstring('module', node)
  File "/usr/lib/python3/dist-packages/pylint/checkers/base.py", line 1633, in _check_docstring
    func = utils.safe_infer(node.body[0].value.func)
  File "/usr/lib/python3/dist-packages/pylint/checkers/utils.py", line 826, in safe_infer
    next(inferit)
RuntimeError: generator raised StopIteration
```