PR: Add warning for deprecated/EoL Qt5 & PyQt5/PySide2 versions to fix #284

[CAM-Gerlach]

As discussed in #284 , adds a DeprecationWarning on QtPy 1.x for Qt5/PyQt5 versions below 5.9 and PySide2 versions below 5.12.

@dalthviz the merge to 1.x will be rather non-trivial (replacing distutils with packaging, changing the warning type, fixing merge conflicts, and possibly a tweak to the message to include users as well, etc) so I'm happy to take care of that too, if you'd like.

Closes #284

[dalthviz]

Thanks @CAM-Gerlach for the help with this! I left some questions/suggestions. Regarding the merge with master, sounds go to me if do it (after merging this then you can go ahead with that 👍)

[ccordoba12]

Looks good to me, thanks @CAM-Gerlach for your work on this!

[dalthviz]

Thanks @CAM-Gerlach for your help with this!

[CAM-Gerlach]

Thanks @dalthviz and @ccordoba12 for the reviews and feedback! Taking care of the master merge now.

[CAM-Gerlach]

Seems like there's some weird behavior with Win 10 + PyQt 5.9 + conda + Python 3.6, where the tests pass, but when we run the command to save the path for the test coverage, which imports QtPy a second time, QT_VERSION is not defined, which means that all the if blocks fell through or ImportErrored. I opened PR #292 which fixes the proximate and secondary problems, and will investigate locally to see if I can reproduce what could point to another underlying issue.