Add __future__ imports to all pytest modules

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

_pytest/_code/__init__.py

@@ -1,4 +1,5 @@
 """ python inspection/code generation API """
+from __future__ import absolute_import, division, print_function
 from .code import Code  # noqa
 from .code import ExceptionInfo  # noqa
 from .code import Frame  # noqa

_pytest/_code/_py2traceback.py

@@ -2,6 +2,7 @@
 # CHANGES:
 # - some_str is replaced, trying to create unicode strings
 #
+from __future__ import absolute_import, division, print_function
 import types
 
 def format_exception_only(etype, value):

_pytest/_code/code.py

@@ -1,3 +1,4 @@
+from __future__ import absolute_import, division, print_function
 import sys
 from inspect import CO_VARARGS, CO_VARKEYWORDS
 import re

_pytest/_code/source.py

@@ -1,4 +1,4 @@
-from __future__ import generators
+from __future__ import absolute_import, division, generators, print_function
 
 from bisect import bisect_right
 import sys