Remove support for PySide and PyQt4 from CI

[mdickinson]

Update the CI builds and etstool.py

• Don't support PyQt4 and PySide; only PyQt5 and PySide2
• Remove CI tests for PyQt4
• Add CI tests for PyQt5 (from EDM) and PySide2 (from pip)

xref: https://github.com/enthought/pyface/pull/512/files

[kitchoi]

This mapping will be inconsistent with traitsui and pyface, where the mapping is "pyqt": {"pyqt5"} instead. It might add to the cognitive load for developers working across multiple ETS projects if envisage chooses a different naming.

I too preferred "pyqt5" in the toolkit name, but I deemed that opinion subjective. Afterall, this is only affecting developers' tools so this is easy to change one way or another.

[mdickinson]

Yes, that's a good point. I'll change it. But in that case, for consistency, we should probably be using pyside instead of pyside2. :-)

[mdickinson]

Actually, I think there is a good non-subjective reason to prefer pyqt5 over pyqt, which is that it matches the names of the corresponding EDM and PyPI packages.

[mdickinson]

I'm going to stick with the existing pyqt5, both because it matches the previous behaviour on this repo (I'd rather not switch pyqt from being valid and meaning one thing to still being valid and meaning something different) and because it matches the PyPI and EDM names.