Use SPDX license identifier (#507)

Author: cdce8p

pyproject.toml

@@ -10,7 +10,7 @@ version = "4.12.2"
 description = "Backported and Experimental Type Hints for Python 3.8+"
 readme = "README.md"
 requires-python = ">=3.8"
-license = { file = "LICENSE" }
+license = { text = "PSF-2.0" }
 keywords = [
     "annotations",
     "backport",