Update astroid to 2.3.1

[pyup-bot]

This PR updates astroid from 2.3.0 to 2.3.1.

Changelog

2.3.1

============================
Release Date: 2019-09-30

* A transform for the builtin `dataclasses` module was added.

This should address various `dataclasses` issues that were surfaced
even more after the release of pylint 2.4.0.
In the previous versions of `astroid`, annotated assign nodes were
allowed to be retrieved via `getattr()` but that no longer happens
with the latest `astroid` release, as those attribute are not actual
attributes, but rather virtual ones, thus an operation such as `getattr()`
does not make sense for them.

* Update attr brain to partly understand annotated attributes

Close 656

Links

• PyPI: https://pypi.org/project/astroid
• Changelog: https://pyup.io/changelogs/astroid/
• Repo: https://github.com/PyCQA/astroid