Add SPDX license identifier

[cdce8p]

Prepare for PEP 639 license expressions.

Setuptools v77 added full support for it. However that version requires Python >=3.9. Until 3.8 is dropped, the next best thing would be to use the legacy license.text field with the correct SPDX license identifier.

All that's needed later would be

-license.text = "BSD-3-Clause"
+license = "BSD-3-Clause"

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files
https://spdx.org/licenses/BSD-3-Clause.html

[cdce8p]

There hasn't been any activity here - closing it. Please fell free to reuse it.