PR: Add a warning for developers still running legacy Qt4-based APIs

[CAM-Gerlach]

We will now warn developers (via a DeprecationWarning, so they have to be running Python with -X dev, -W default or -W error, or import QtPy in their __main__ script) when they are using long-deprecated, soon to be removed Qt4 based APIs (PySide and PyQt4), either via their explictly set preference, via import, or via a default fallback if installed, all of which will be unsupported come QtPy 2.0.

Resolves #261

[CAM-Gerlach]

Looks like the CIs are not running at all. I'll see if I can fix that...

[CAM-Gerlach]

The issue is the old CI job, before my improvements, only runs on master. I'll modify it here so it triggers as well (so long as it works being in a fork from my repo, which is the thing I had trouble with before, otherwise I'll have to do it in another branch/PR here).

[CAM-Gerlach]

(So, incidentally, PR #281 skipped the tests and checks completely)

[ccordoba12]

Looks good to me, thanks @CAM-Gerlach!

[CAM-Gerlach]

I amended it into the same commit, to hopefully make it a little easier when merging to master (since adding the warning and the release branch pattern will need to be dropped when merging; LEGACY_APIS can stay.

[CAM-Gerlach]

@ccordoba12 Want me to merge when the CIs pass, or should we wait for @dalthviz ?

[dalthviz]

Is there any need to merge 1.x with master after this PR @CAM-Gerlach ? In the merge we will need to readd PyQT4_API and PYSIDE_API or change LEGACY_APIS to be ['pyqt', 'pyqt4', 'pyside'] while the rest of the changes are dropped, right?

[ccordoba12]

@dalthviz, you can still merge with master and drop all changes present here (because they don't apply) by doing an empty commit for the merge.

[CAM-Gerlach]

Ah, I somehow forgot that LEGACY_APIS also required having those other constants, which probably aren't harmful but realistically aren't really needed, so I guess a just doing a null merge is fine (to still keep the branches in sync of course). Thanks for taking care of it, @dalthviz .