Use new-style license metadata for Flit itself

Author: takluyver

pyproject.toml

@@ -16,7 +16,8 @@ dependencies = [
 ]
 requires-python = ">=3.8"
 readme = "README.rst"
-license = {file = "LICENSE"}
+license = "BSD-3-Clause"
+license-files = ["LICENSE"]
 classifiers = ["Intended Audience :: Developers",
     "License :: OSI Approved :: BSD License",
     "Programming Language :: Python :: 3",