Set license and license-files key in project metadata to follow PEP639 (#75)

* Set license and license-files key in project metadata to follow PEP639

Adhere to https://peps.python.org/pep-0639 to specify Apache-2.0 license in SPDX format. Works on setuptools>=77.

* Bump actions/checkout to v4, actions/setup-python to v5

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

Bumps [actions/setup-python](https://github.com/actions/setup-python) from 4 to 5.
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](actions/setup-python@v4...v5)

Author: weiji14

pyproject.toml

@@ -1,12 +1,13 @@
 [build-system]
-requires = ["setuptools", "versioneer[toml]"]
+requires = ["setuptools>=77", "versioneer[toml]"]
 build-backend = "setuptools.build_meta"
 
 [project]
 name = "cupy-xarray"
 description = "Interface for using cupy in xarray, providing convenience accessors."
 authors = [{name = "cupy-xarray developers"}]
-license = {text = "Apache License"}
+license = "Apache-2.0"
+license-files = ["LICENSE"]
 readme = "README.md"
 requires-python = ">=3.10"
 classifiers = [