setup: update to SPDX license expression

[oliver-sanders]

Spotted in conda-build output:

https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=1295875&view=logs&j=7b6f2c87-f3a7-5133-8d84-7c03a75d9dfc&t=9eb77fd2-8ddd-5444-8fc0-71cb28dcb736&l=2369

          Please consider removing the following classifiers in favor of a SPDX license expression:

          License :: OSI Approved :: GNU General Public License v3 (GPLv3)

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

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

If we decide to this this, please create issues on the other cylc/rose repos.