Use a valid SPDX identifier as license classifier

This helps automatic license checkers like pip-licenses to identify the right license for this project

Author: ecederstrand

pyproject.toml

@@ -7,6 +7,7 @@ name = "cons"
 dynamic = ["version"]
 description = "An implementation of Lisp/Scheme-like cons in Python."
 readme = "README.md"
+license: "LGPL-3.0"
 license-files = ["LICENSE.txt"]
 authors = [
     { name = "Brandon T. Willard", email = "brandonwillard+cons@gmail.com" },