adapt LICENSE metadata to SPDX expression

https://packaging.python.org/en/latest/specifications/license-expression/

Author: shimizukawa

pyproject.toml

@@ -7,7 +7,8 @@ authors = [
 description = "Sphinx utility that make it easy to translate and to apply translation."
 readme = "README.rst"
 requires-python = ">=3.9"
-license = {file = "LICENSE"}
+license = "BSD-2-Clause"
+license-files = ["LICENSE"]
 dependencies = [
     "click>=8.0.0",
     "babel>=2.9.0",
@@ -16,7 +17,6 @@ dependencies = [
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
-    "License :: OSI Approved :: BSD License",
     "Topic :: Documentation",
     "Topic :: Documentation :: Sphinx",
     "Topic :: Software Development",