Bump astroid from 1.6.4 to 2.0.4

Bumps [astroid](https://github.com/PyCQA/astroid) from 1.6.4 to 2.0.4.
- [Release notes](https://github.com/PyCQA/astroid/releases)
- [Changelog](https://github.com/PyCQA/astroid/blob/master/ChangeLog)
- [Commits](pylint-dev/astroid@astroid-1.6.4...astroid-2.0.4)

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

requirements.txt

@@ -1,4 +1,4 @@
-astroid==1.6.4
+astroid==2.0.4
 colorama==0.3.9
 dj-database-url==0.5.0
 Django==2.0.5