Make it work also with PySide2

[ramirahikkala]

Some prefer PySide2 over PyQt because of different licences. Api between those are almost the same. With conditional import this should work with both.

[fbjorn]

Hi @ramirahikkala , thank you for your interest in this tiny library.
Let's also remove PyQt5 as a "requirement" from setup.py then. Moreover, this param is incorrect and was mixed up with install_requires probably. But let's not confuse people from now on