Update license information in pyproject.toml

[aytekinar]

Make the licensing of the project PEP 639 compatible.

See the user guide, respective PEP, and the support in setuptools.

Closes #125.
Fixes #362.

[dmholtz]

+1 for this change. This resolves deprecation warnings with modern versions of setuptools

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Please use a simple string containing a SPDX expression for
`project.license`. You can also use `project.license-files`. (Both
options available on setuptools>=77.0.0).

By 2027-Feb-18, you need to update your project and remove
deprecated calls or your builds will no longer be supported.

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

[sylvesterkaczmarek]

One compatibility bit looks missing here. The new PEP 639 license / license-files form needs Setuptools 77+, but build-system.requires still allows setuptools>=62.4. So an otherwise allowed older build backend can reject this pyproject.toml. I think the Setuptools floor should move to 77+ with this change.