Explicitly close read and write of Popen to latex

This avoids a warning in the test suite on python3.4. When reading
and writing from directly from stdout and stdin of the process they
should be explicitly closed afterwards.
Close framesink in non file based animation too.
Merge pull request #3309 from jenshnielsen/close_files_pgf

MNT : Explicitly close read and write of Popen process (latex)
Remove usage of raw strides member in _backend_gdk.c
Fix _backend_gdk.c
Merge pull request #3665 from jkseppan/fix-gdk-strides

BUG : Remove usage of raw strides member in _backend_gdk.c
Python3 pep8 fixes

I am seeing these pep8 failures in the test suite only with python3. Because print isn't a function in python2? Travis does not run pep8 on python3 so I only see this fail locally