Apply PEP 639

Prompted by this post https://bsky.app/profile/hynek.me/post/3lc5n7g6kr22f

Ref: python-attrs/attrs#1377

Author: nicoddemus

pyproject.toml

@@ -13,7 +13,8 @@ keywords = [
     "test",
     "unittest",
 ]
-license = { text = "MIT" }
+license = "MIT"
+license-files = [ "LICENSE" ]
 authors = [
     { name = "Holger Krekel" },
     { name = "Bruno Oliveira" },
@@ -27,7 +28,6 @@ requires-python = ">=3.9"
 classifiers = [
     "Development Status :: 6 - Mature",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: MIT License",
     "Operating System :: MacOS",
     "Operating System :: Microsoft :: Windows",
     "Operating System :: POSIX",