Update license specifier in pyproject.toml

[zm711]

Please note that as of PEP639 we need to specify our license differently

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

min version of Setuptools for this is 77 so we should discuss whether we want to update our build dependency to that.

[h-mayorquin]

I am +1 for just setting a higher floor for setuptools. This should not affect users as the backend is built in isolation and we don't really use any new feature.

Or, if we are ambitious we could move to hatchling which I would prefer.