Bump astroid from 1.6.3 to 2.1.0

Bumps [astroid](https://github.com/PyCQA/astroid) from 1.6.3 to 2.1.0.
- [Release notes](https://github.com/PyCQA/astroid/releases)
- [Changelog](https://github.com/PyCQA/astroid/blob/master/ChangeLog)
- [Commits](pylint-dev/astroid@astroid-1.6.3...astroid-2.1.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements.txt

@@ -1,4 +1,4 @@
-astroid==1.6.3
+astroid==2.1.0
 attrs==18.2.0
 certifi==2018.4.16
 chardet==3.0.4