Improve licence metadata (PEP 639) (#322)

Author: mwtoews

pyproject.toml

@@ -3,7 +3,8 @@ name = "spin"
 requires-python = ">=3.10"  # Oldest non-EOL Python here
 description = "Developer tool for scientific Python libraries"
 readme = "README.md"
-license = {file = "LICENSE"}
+license = "BSD-3-Clause"
+license-files = ["LICENSE"]
 maintainers = [
   {name = "Scientific Python", email = "spin@discuss.scientific-python.org"}
 ]
@@ -12,7 +13,6 @@ classifiers = [
   "Programming Language :: Python",
   "Intended Audience :: Developers",
   "Intended Audience :: Science/Research",
-  "License :: OSI Approved :: BSD License",
   "Operating System :: OS Independent",
   "Programming Language :: Python :: 3",
   "Programming Language :: Python :: 3.10",