Declare the license as a PEP 639 expression in pyproject.toml

A license= keyword in setup.py still lands in the legacy free-text License
field, so the published metadata reads "License: Apache-2.0" rather than
"License-Expression: Apache-2.0". Only a [project] license in pyproject.toml
produces the expression.

Adds a pyproject.toml declaring the build backend, the license and the license
file, and leaves every other field dynamic so setup.py stays the single place
scripts/update-client.sh rewrites at release time.

Drops the now-redundant license= keyword from the three setup scripts;
pyproject.toml overrides it, and setuptools warns when both are set.

Author: chala2001

pyproject.toml

@@ -0,0 +1,36 @@
+# Copyright 2026 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+[build-system]
+requires = ["setuptools>=77"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "kubernetes"
+license = "Apache-2.0"
+license-files = ["LICENSE"]
+# Everything else is still declared in setup.py, which scripts/update-client.sh
+# rewrites on every release.
+dynamic = [
+    "version",
+    "description",
+    "readme",
+    "requires-python",
+    "dependencies",
+    "optional-dependencies",
+    "classifiers",
+    "keywords",
+    "authors",
+    "urls",
+]

setup-asyncio.py

@@ -39,7 +39,6 @@
     description="Kubernetes asynchronous python client",
     author_email="",
     author="Kubernetes",
-    license="Apache-2.0",
     url="https://github.com/kubernetes-client/kubernetes/aio",
     keywords=["Swagger", "OpenAPI", "Kubernetes"],
     install_requires=REQUIRES,

setup-release.py

@@ -60,7 +60,6 @@
     description="Kubernetes python client",
     author_email="",
     author="Kubernetes",
-    license="Apache-2.0",
     url="https://github.com/kubernetes-client/python",
     keywords=["Swagger", "OpenAPI", "Kubernetes"],
     install_requires=REQUIRES+REQUIRES_ASYNCIO,

setup.py

@@ -53,7 +53,6 @@
     description="Kubernetes python client",
     author_email="",
     author="Kubernetes",
-    license="Apache-2.0",
     url="https://github.com/kubernetes-client/python",
     keywords=["Swagger", "OpenAPI", "Kubernetes"],
     install_requires=REQUIRES,