Remove from __future__ import absolute_import.

Except in:
- docs/writing-bots-guide.md, because bots are supposed to be Python 2
  compatible
- puppet/zulip_ops/files/zulip-ec2-configure-interfaces, because this
  script is still on python2.7
- tools/lint
- tools/linter_lib
- tools/lister.py

For the latter two, because they might be yanked away to a separate repo
for general use with other FLOSS projects.

manage.py

@@ -1,6 +1,4 @@
 #!/usr/bin/env python3
-from __future__ import absolute_import
-
 import os
 import sys