downloads: annotate readline SPDX license as GPL-3.0-only

The old `GPL-3.0` identifier is deprecated as of SPDX 3.0. Let's
stay modern.

Author: indygreg

pythonbuild/downloads.py

@@ -200,7 +200,7 @@
         "sha256": "7589a2381a8419e68654a47623ce7dfcb756815c8fee726b98f90bf668af7bc6",
         "version": "8.1.2",
         "library_names": ["readline"],
-        "licenses": ["GPL-3.0"],
+        "licenses": ["GPL-3.0-only"],
         "license_file": "LICENSE.readline.txt",
     },
     "setuptools": {