Use setuptools license_expression for the license metadata

[chala2001]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Follow-up to #2687, as requested there.

#2687 set the license to the SPDX identifier Apache-2.0, but a plain license=
keyword lands in the legacy free-text License field. setuptools'
license_expression= keyword produces the PEP 639 License-Expression field
instead, so that is what the three setup scripts now use.

Metadata built with setuptools 84.0.0:

before:  License: Apache-2.0
after:   License-Expression: Apache-2.0

Checked all three scripts, and the distribution names are unchanged:

setup.py           Name: kubernetes        License-Expression: Apache-2.0
setup-release.py   Name: kubernetes        License-Expression: Apache-2.0
setup-asyncio.py   Name: kubernetes.aio    License-Expression: Apache-2.0

License-File: LICENSE is still there, and the legacy License field is gone.
pip install -e ., which is what tox does via usedevelop = True, still works.

Which issue(s) this PR fixes:

Follow-up to #2687, no separate issue.

Special notes for your reviewer:

license_expression= needs setuptools 77 or newer. Older versions ignore it with a
warning rather than failing, so an old build environment gets the same metadata it
gets today.

An earlier version of this PR added a pyproject.toml with [project] license.
That worked, but [project] name cannot be dynamic, so it renamed the
setup-asyncio.py build from kubernetes.aio to kubernetes. The keyword route
you asked for avoids that entirely and is three lines instead of a new file.

Does this PR introduce a user-facing change?

Package metadata now declares the license using the PEP 639 `License-Expression` field.

[yliaog]

please use setuptools' license_expression= keyword in setup() instead, thanks.

[chala2001]

Done - switched to license_expression= in the three setup scripts and dropped the pyproject.toml. Verified License-Expression: Apache-2.0 in all three builds, and setup-asyncio.py keeps its kubernetes.aio name this way. Thanks..

[yliaog]

thanks for the PR

/lgtm
/approve

[kubernetes-prow]

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