build: Fix obsolete license format in pyproject.toml

Author: adrienverge

pyproject.toml

@@ -5,7 +5,7 @@ A fake but stateful Stripe server that you can run locally, for \
 testing purposes."""
 readme = {file = "README.rst", content-type = "text/x-rst"}
 requires-python = ">=3.9"
-license = {text = "GPL-3.0-or-later"}
+license = "GPL-3.0-or-later"
 authors = [{name = "Adrien Vergé"}]
 classifiers = [
     "Development Status :: 5 - Production/Stable",