Removed Python legacy __future__ imports (#8697)

There are still some left, but the `modification_check` test fails. Some are still in `tools`, and they can probably be removed as well. `with_statement` was introduced officially in Python 2.5, so there's really old stuff in the code base.

tools/pipstrap.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 """Uses pip to upgrade Python packaging tools to pinned versions."""
-from __future__ import absolute_import
 import os
 
 import pip_install