Update repo license metadata

Switch `project.license` to an SPDX expression and add `license-files`
in the repository `pyproject.tmol` to avoid setuptools license
deprecations.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

pyproject.toml

@@ -9,7 +9,8 @@ build-backend = "setuptools.build_meta"
 name = "frequenz-repo-config"
 description = "Frequenz repository setup tools and common configuration"
 readme = "README.md"
-license = { text = "MIT" }
+license = "MIT"
+license-files = ["LICENSE"]
 keywords = [
   "config",
   "frequenz",
@@ -29,7 +30,6 @@ keywords = [
 classifiers = [
   "Development Status :: 3 - Alpha",
   "Intended Audience :: Developers",
-  "License :: OSI Approved :: MIT License",
   "Programming Language :: Python :: 3",
   "Programming Language :: Python :: 3 :: Only",
   "Topic :: Software Development :: Libraries",