Fix a package build deprecation warning

[kurtmckee]

This manifests when building the package:

pip install build
python -m build

and the error is:

SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated

********************************************************************************
Please use a simple string containing a SPDX expression for `project.license`.
You can also use `project.license-files`.

This deprecation is overdue, please update your project and remove deprecated
calls to avoid build errors in the future.

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************

[kurtmckee]

Rebased, resolved merge conflicts, and force-pushed

Hello @theskumar, I hope you're doing well! Would you take a look at this PR when you have an opportunity? It resolves an package build warning. Thanks!

[theskumar]

Thanks @kurtmckee, and sorry this sat for so long. I reproduced the warning: with setuptools 84.0.0, python -m build prints it 4 times on main. Your change removes all of them, and the wheel now has License-Expression: BSD-3-Clause metadata. We already require setuptools >= 77, so nothing else needs to change.

I pushed two commits on top of yours, without changing your commits:

• Merge main: fixes the CHANGELOG.md conflict. 1.2.4 shipped in the meantime, so your entry's old section had become the 1.2.4 release notes.
• CHANGELOG: moves your entry under a new [Unreleased] / ### Fixed and adds the by [@kurtmckee] in [#648] credit with the link definitions, matching the file's format.

I'll merge once CI is green.