[#914] Drop the setuptools<82 build-time pin

[eoyilmaz]

Summary

• Replaces setuptools<82 with setuptools>=77 in pyproject.toml's [build-system] requires. The <82 ceiling was introduced because "some functionalities DisplayCAL currently requires are deprecated" (util/ez_setup.py: pkg_resources is removed from setuptools 82+ #667), but empirical testing shows the actual setuptools 82 breaking change was pkg_resources removal, which nothing in this build path touches (only the separately-slated-for-deletion util/ez_setup.py uses it, and it's never imported during a build). setuptools 83.0.0 still vendors a fully working _distutils package, so distutils.core, distutils.util, distutils.command.install, and distutils.filelist all keep working unpinned. The >=77 floor is kept since that's when SPDX license/license-files support (needed by [Packaging] Update project.license to an SPDX string before setuptools' 2027-02-18 deadline #915) landed.
• Cleans up DisplayCAL/freeze.py, which turned out to duplicate both anti-patterns [Packaging] MANIFEST.in gets silently regenerated as a side effect of building #912 and [Packaging] Build metadata is duplicated between pyproject.toml and DisplayCAL/setup.py, and has already drifted #913 already fixed in DisplayCAL/setup.py, but was never itself touched:
  • Its attrs dict carried the same dead name/classifiers/description/license/long_description/entry_points metadata, even more clearly dead here since setup(**attrs) is commented out and only console/windows/data_files/zipfile/options are ever read back out of it.
  • It unconditionally rewrote MANIFEST.in from scratch on every py2exe freeze, the exact side effect [Packaging] MANIFEST.in gets silently regenerated as a side effect of building #912 removed from DisplayCAL/setup.py.
  • Removed the now-fully-unused distutils.filelist.findall monkeypatch (never exercised, since this file never invokes distutils' sdist/setup()), and swapped the one live distutils.util.get_platform() call for stdlib sysconfig.get_platform() (verified identical output).
• Deliberately did not rewrite the real, working distutils.core/distutils.command.install/distutils.util.change_root usage in DisplayCAL/setup.py, native_build.py, or util/make-site-package-dist.py. setuptools has no announced removal date for its vendored distutils shim (only pkg_resources was pulled abruptly), so that larger rewrite is deferred as a separate follow-up rather than bundled here for no currently-proven benefit.

Verification

• Fresh-clone isolated python -m build (sdist -> wheel-from-sdist) on both Python 3.14 and Python 3.9, resolving unpinned setuptools 83.0.0: succeeds, wheel has 711 files, entry_points.txt/METADATA intact.
• Direct native_build.py install --prefix=... (exercises the real distutils.command.install/change_root/get_platform path) produces an identical install tree with setuptools 83 unpinned vs. the setuptools 81 pinned baseline on develop.
• Full serial pytest run: 2576 passed, 21 skipped, 0 failures.

Test plan

• CI green across all workflows (Windows/macOS/Linux/Flatpak/pytest)
• Nightly build workflow succeeds