Remove Python 2.7, 3.5 testing, add PyQt 5

[jvkersch]

as it says on the tin.

Note: this means that we effectively don't test with wxpython and pyside any more.

@rahulporuri Do you know of any other toolkits that are supported under Python 3.6?

[corranwebster]

You should be able to get things to run with pip-installed pyside2, with a bit of luck. Have a look at the TraitsUI and Pyface etstools.py to see how we do it there.

[rahulporuri]

We can make the same changes on appveyor too, right?

[rahulporuri]

LGTM!

[jvkersch]

@rahulporuri Thanks for the quick turnaround on reviewing. I think this now works, care to give it a final look? I had to make one non-trivial change for pyside2: under pyside2, instantiating a QFrame with a keyword argument size does not work. The workaround is to omit the size argument, and to resize the window afterwards. You can see the original failure with the following code snippet:

from pyface.qt import QtCore, QtGui
app = QtGui.QApplication([])
size = QtCore.QSize(10, 10)
f = QtGui.QFrame(size=size)  # complains about missing setSize method

The commit history is cluttered, so we should merge this with squash-merge.

[jvkersch]

Ha, you beat me to it :)

[rahulporuri]

yep yep I wasn't sure why QFrame.setSize doesn't exist on PySide2 but exists on PyQt5. I didn't look closely enough at the PyQt5 docs but the PySide2 docs for QFrame didn't mention any setSize