Improve example docs for setup.cfg and pyproject.toml

[mwtoews]

This PR improves the example documentation for both setup.cfg and pyproject.toml.

Changes to both examples:

• Add an author and email, as these are commonly used (an alternative fictional person could be nominated, if a better one exists)
• Use the SPDX short identifier BSD-3-Clause, which aligns with PEP 639

Other:

• With setup.py only, add python_requires to example, so that it is compatible to pyproject.toml

[abravalheri]

Thank you @mwtoews.

My understanding is that, so far, the license text is free form. Once PEP 639 is approved, then there will be a separated field for them... Nevertheless it should not hurt to have SPDX in the examples (as long as users don't expect it to work before PEP 639 is implemented/with the wrong fields).