pyzfs: update license tags/classifiers

[robn]

[Sponsors: TrueNAS]

Motivation and Context

I got annoyed at this noise in the build:

/usr/lib/python3/dist-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: Apache Software License

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

!!
  self._finalize_license_expression()

Description

The standard for package license metadata[1] is a SPDX identifier in the the license and that's all. So, updating that, remove the deprecated license classifier, and adding a tag at the top of the file for spdxcheck to find.

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

How Has This Been Tested?

Built, observed noise, changed it, built again, observed (relative) silence.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.