Update licensing metadata to adhere to PEP 639.

Author: freakboy3742

pyproject.toml

@@ -11,7 +11,10 @@ name = "rubicon-objc"
 description = "A bridge between an Objective C runtime environment and Python."
 readme = "README.rst"
 requires-python = ">= 3.9"
-license.text = "New BSD"
+license = "BSD-3-Clause"
+license-files = [
+    "LICENSE",
+]
 authors = [
     {name="Russell Keith-Magee", email="russell@keith-magee.com"},
 ]
@@ -26,7 +29,6 @@ keywords = [
 classifiers = [
     "Development Status :: 4 - Beta",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: BSD License",
     "Programming Language :: Objective C",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.9",