python/docker: finalize pyproject build-system and SPDX license

- Bump setuptools floor to >=61 in pyproject.toml with rationale comments.
- Declare build-backend explicitly; keep classic scikit-build.
- Switch setup.py license to SPDX expression (GPL-3.0-or-later) and remove
  deprecated license classifier, keeping only non-license classifiers.

python/pyproject.toml

@@ -1,11 +1,14 @@
 [build-system]
+# NOTE: Setuptools >=61 fully supports pyproject-based builds and is the common
+# floor for projects that may later adopt PEP 621 metadata. Keeping scikit-build
+# classic for now; full migration is tracked in the TODO below.
 requires= [
-  "setuptools>=42",
+  "setuptools>=61",
   "scikit-build>=0.13",
   "cmake>=3.15",
   "ninja"
 ]
-# NOTE (2025-09-12): Explicitly declare the backend used by `python -m build`.
+# Used by `python -m build` (PEP 517).
 build-backend = "setuptools.build_meta"
 
 # TODO (modernization): Consider migrating to scikit-build-core and PEP 621.

python/setup.py.in

@@ -35,11 +35,13 @@ setup(
             ],
     package_data={'opm' : ['$<TARGET_FILE_NAME:simulators>']},
     include_package_data=True,
-    license='Open Source',
+    # SPDX license expression to replace deprecated license classifier usage.
+    license='GPL-3.0-or-later',
     install_requires=requires,
     python_requires='>=3.8',
+    # NOTE (2025-09-12): Remove deprecated license classifier in favor of
+    # SPDX license expression above. Keep non-license classifiers only.
     classifiers=[
         "Programming Language :: Python :: 3",
-        "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
     ],
 )