PR: Make warning usage consistant and refine messages

[CAM-Gerlach]

Currently, as noted in issue #395 , a generic RuntimeWarning is emitted when falling back to a non-selected binding, instead of QtPy's own PythonQtWarning subclass. Furthermore, the text could perhaps be clarified a bit. I also noticed a few other issues when making this changed, which I fixed.

Therefore, this PR:

• Emits a PythonQtWarning instead of a generic RuntimeWarning when falling back to an unselected binding, consistent with the other warnings and errors that use QtPy-specific subclasses
• Has PythonQtWarning inherit from RuntimeWarning instead of a generic Warning , to be consistent with all the other errors and warnings and with what RuntimeWarning is meant for, and ensure the previous replacement is fully backwards compatible
• Tweaks the message text of the former warning to be clearer and more grammatically correct
• Fixes the outdated and incorrect docstrings of PythonQtWarning and PythonQtError
• Updates both warning usages to modern code formatting, f-strings and to use !r instead of hardcoded quotes

[CAM-Gerlach]

(Oops, fell asleep before I went back and hit the ready for review button)

[ccordoba12]

Two small suggestions for you @CAM-Gerlach, otherwise looks good to me.

[ccordoba12]

Looks good to me now, thanks @CAM-Gerlach!

[dalthviz]

Thanks @CAM-Gerlach !

[dalthviz]

Also @CAM-Gerlach should this close #395 ?

[CAM-Gerlach]

It may help with it some, but it isn't clear that it would resolve the actual issue the user experienced, since from the very limited information provided it is not at all clear what that actually is. I suggest we wait at least a couple weeks to see if they respond. Also, you mentioned documenting the error/warning hierarchy somewhere, which is a good idea to do at some point.