PR: Upgrade packaging to avoid legacy/deprecated behavior and follow PEP 517

[CAM-Gerlach]

Currently, the package is using the deprecated legacy builder and calling setup.py directly, which is causing warnings and will eventually go away completely. This migrates it to the (more) modern PEP 517 setuptools builder and setup.cfg, with a setup setup.py for backward compatibility. This also makes one change to how the version is handled to avoid having to import the package or execute any dynamic code to get the version statically, which shouldn't affect anything else as _version is marked as internal use only (with leading _). I've compared the wheels and sdists and, aside from the minor updates to the metadata made in a separate commit, they are byte for byte identical.

• Update packaging for initial compatibility with PEP 517 and PEP 518
  • Add pyproject.toml
  • Update CIs
  • Update RELEASE.md accordingly
• Use modern declarative setup.cfg instead of legacy dynamically-executed setup.py
  • Migrate setup.py metadata to setup.cfg
  • Fix/avoid encoding issues on open()
  • Run setup-cfg-format
  • Add stub setup.py for backward compat with legacy tools
  • More reliable static version handling
  • Update RELEASE,md accordingly
• Add a few new bits of setup.cfg metadata
  • Add additional appropriate Trove tags
  • Add project urls
  • Add test extra
• Update RELEASE,md to follow modern PEP 517/518 best practices

Resolve #270

[dalthviz]

Thanks @CAM-Gerlach ! I left a suggestion and a comment/question. Besides that this LGTM 👍

Also, thank you @Kant-ain for the misspelling fix suggestion !

[ccordoba12]

Thanks @CAM-Gerlach for this! I left a couple of comments, otherwise looks good to me.

[ccordoba12]

Thanks @CAM-Gerlach for your work on this!

[dalthviz]

Thanks @CAM-Gerlach !!

[CAM-Gerlach]

Thanks for your feedback and bearing with me on this @ccordoba12 and @dalthviz !