Add __future__ imports to all pytest modules

This prevents silly errors from creeping in Python 2 when testing in Python 3

testing/code/test_source.py

@@ -1,6 +1,7 @@
 # flake8: noqa
 # disable flake check on this file because some constructs are strange
 # or redundant on purpose and can't be disable on a line-by-line basis
+from __future__ import absolute_import, division, print_function
 import sys
 
 import _pytest._code