chore: Migrate from pep517 to python-build

[matthewfeickert]

Switch from using the now deprecated pep517 to build sdist and wheels to the new python-build library. c.f. scikit-hep/pyhf#1066 for more context.

(I forgot to migrate this over into PR #29 once we learned about this for pyhf).

Suggested squash and merge commit message

* Migrate from the deprecated pep517.build to python-build CLI for builds for packaging
   - c.f. https://github.com/pypa/pep517/pull/83
* Remove check stage as done implicitly with python-build

[matthewfeickert]

@dguest This is ready for review (sorry I forgot to update PR #29 with this when I learned about the change last week).