[ADD] added PySide compability

[holg]

Added import statements for PySide for PySide comp ability

[wiredfool]

I'm worried about testing this -- PySide appears to be a pretty big set of dependencies to install on travis each time, and they don't ship binaries for linux.

[holg]

Well i agree, specially it is not easy to have those different Versions and kind of QT stuff,
pillow starts to test for PyQt5, then looks up PyQt4, so now this commit adds another to lookup PySide,
i would not think to much about testing this, in the end no source code in pillow should be changed, since PyQt4 and PySide do it the same way (regarding to the needs of pillow), so testing the one would be enough, i am always trying to keep my code compatible for both, in the end PySide has some advantages for using in cx_freeze and other packaging, whereas at all i would prefer PyQt5

[hugovk]

At the moment we're only testing PyQt4 on the 2.7_with_system_site_packages environment.

[holg]

ok, so how do you check PyQt5 ?
anyway i forgot to also adapt Tests/test_image_qt.py to pass tests in PySide environment

[wiredfool]

We don't continually test PyQt5, and I'm not all that happy about that situation.