chore: Add license file metadata (#291)

Author: bkmgit

pyproject.toml

@@ -6,7 +6,10 @@ description = "The build backend used by PDM that supports latest packaging stan
 authors = [
   { name = "Frost Ming", email = "me@frostming.com" }
 ]
-license = {text = "MIT"}
+license = "MIT"
+license-files = [
+  "LICENSE",
+]
 requires-python = ">=3.9"
 readme = "README.md"
 keywords = ["packaging", "PEP 517", "build"]