Python wheel shouldn't be demanded for local bulids from source via setuptools

[alerque]

See previous iterations of this issue on other projects for details:

• https://github.com/googlefonts/glyphsLib/issues/701
• https://github.com/fonttools/ufoLib2/issues/156
• https://github.com/typemytype/booleanOperations/pull/63

This one I'm even less sure how to go about fixing because a different shim to setuptools seems to be used, but here's the bit of code:

https://github.com/unified-font-object/ufoNormalizer/blob/6d2e69ce056c24d60d397881d56f018cad2f2475/pyproject.toml#L2

[benkiel]

Happy to review a PR for this, unfortunately I don’t currently have the bandwidth to dig into this.