Port packaging from deprecated legacy builder to current PEP 517 standards

I was already planning to do this following #262 being merged, as I’ve done for many other packages. However, with setuptools 58.3.0, which we are now getting a formal DeprecationWarning when installing the package (which correctly fails the CIs, due to -W error), and it will soon stop working entirely.

The issue is due to using the old legacy setup.py backend that has been discouraged for a number of years, to the modern pyproject.toml-based setuptools backend that conforms to the PEP 517 standard. Its straightforward to migrate and should no negative/backward compatibly effect on users (other than improving the robustness of building and installing the package), as PEP 517-conformant builds actually allows installing even on machines that don’t have an otherwise compatible installation of setuptools with suitably recent pip, and I’ll still include a backward-compatibly setuptools shim for older ones, so it’ll still work as before for older non-PEP 517 pip/wheel/setuptools. And of course, this is all now fully tested end to end on all platforms and versions in our CIs, following #262 being merged.

So, I’ll migrate the config from the dynamic setup.py to the determistic setup.cfg and simplify it in the process (e.g. no need for bespoke code to get the version, readme, etc, as that is all handled by setuptools at build time), add a setup.py shim for backward compatibility, and add a pyproject.toml with the basic PEP 517 config, all following the standard PyPA recommendations as now being followed by the rest of the ecosystem. I’ll also update the CIs and the docs where applicable.

_Originally posted by @CAM-Gerlach in https://github.com/spyder-ide/qtpy/issues/268#issuecomment-955067333_

[ccordoba12]

I’m ok with this too.

[CAM-Gerlach]

NB, we’ll need to do this in Spyder itself at some point soon for the same reasons, as well as other porting (e.g. removing the uses of the long-deprecated and soon to be removed distutils, so it won’t raise a DeprecationWarning in Python 3.10 and doesn’t break on 3.12, when it will be removed). so this will be good to get familiar with now. I can certainly help with that too, but good to take a much smaller first step doing it with QtPy (and other less complex/critical packages) first.