PEP 639 not being supported by all backends yet

[befeleme]

          I think updating the guide might have been a little early, as not all backends (namely setuptools and flit-core, which are mentioned, along with others that are not mentioned like poetry-core) support PEP 639 yet. Maybe there should at least be a warning?

See pypa/packaging-problems#870

Originally posted by @henryiii in #1662 (comment)

Steps in my head:

• explicitly state which build backends support PEP 639
• keep the bits of the older guidelines to follow by people using the other build backends

Pages to update:

• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/ (describe the double way of declaring metadata)
• https://packaging.python.org/en/latest/tutorials/packaging-projects/ (differentiate the example pyproject.toml between build-backends)
• https://packaging.python.org/en/latest/guides/licensing-examples-and-user-scenarios/ (add a warning about build backends supporting the new specs)

I intend to work on this during the next week.