chore: Python 3.12 is the floor

netOrk no longer supports 3.10 or 3.11 anywhere. The actions run on the
runner's job container, catthehacker/ubuntu:act-24.04, whose python3 is
3.12; CI already tests on 3.12. Refs NAPALM/driver-standard#6.

pyproject.toml

@@ -2,7 +2,7 @@
 name = "image-tools"
 version = "0.1.0"
 description = "Judge, sign and verify netOrk container images"
-requires-python = ">=3.11"
+requires-python = ">=3.12"
 license = "MIT"
 
 [build-system]