Drop support for PySide

[fgallaire]

PySide has been created by Nokia, Microsoft killed Nokia and so PySide and we only test PyQt in the Travis CI.

So, should we drop PySide support ?

[PierreRaybaut]

I think I haven't tested formlayout with PySide since 2009... so "PySide support" is probably overrated 😄.

Anyway, I'm haven't been following the PySide project for a while now.
So => no opinion.