pyproject: switch to PEP 639 license metadata

Use a SPDX license expression to declare our licensing, add links to our
license files, and drop the deprecated license Trove classifier.

Without this change, setuptools throws large deprecation warnings claiming
that support for the old syntax will be removed on 2026-Feb-18.  However,
the new syntax for project.license requires setuptools >= 77, so this
change breaks compatibility with setuptools 61-76.  Users installing from
source will be unaffected because the build frontend automatically updates
setuptools if required, but Linux distro packaging disallows such updates,
so this change breaks package builds for many still-supported distro
versions.  To build with older setuptools, distros will need to patch out
the project.license field and reduce the build-system.requires dependency.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

Author: bgilbert

pyproject.toml

@@ -5,14 +5,14 @@ maintainers = [
 ]
 description = "Python interface to OpenSlide"
 readme = "README.md"
-license = {text = "GNU Lesser General Public License, version 2.1"}
+license = "LGPL-2.1-only AND BSD-3-Clause AND MIT AND LicenseRef-Public-Domain"
+license-files = ["COPYING.LESSER", "**/LICENSE.*"]
 keywords = ["OpenSlide", "whole-slide image", "virtual slide", "library"]
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Developers",
     "Intended Audience :: Healthcare Industry",
     "Intended Audience :: Science/Research",
-    "License :: OSI Approved :: GNU Lesser General Public License v2 (LGPLv2)",
     "Operating System :: MacOS :: MacOS X",
     "Operating System :: Microsoft :: Windows",
     "Operating System :: POSIX :: Linux",
@@ -84,5 +84,5 @@ pythonpath = "tests"
 ignore_messages = "(Hyperlink target \".*\" is not referenced\\.$)"
 
 [build-system]
-requires = ["setuptools >= 61.0.0"]
+requires = ["setuptools >= 77.0.0"]
 build-backend = "setuptools.build_meta"