MAINT: fix(license): update pypi license info

[chenrui333]

👋 since the project is using BSD-3-Clause, update the license info on the pypi side.

[Lucas-C]

Good idea.

Apparently License :: OSI Approved :: BSD-3-Clause is not a valid Pypi classifier: https://pypi.org/classifiers/

Seems lile License :: OSI Approved :: BSD License is fine.

[chenrui333]

yeah, good point, I am bit surprised that they still did not support the SPDX id yet. let me update to use BSD License then, thanks!

[Lucas-C]

Merged!