context: fix is_stable_release() definition of "stable"

According to distro-info,  the devel release is "supported". And, the
existing code forgot about ESM.

Hence, fix the definition of stable by (a) including ESM, and (b)
excluding devel.

Author: enr0n

ubuntu_lint/context.py

@@ -96,4 +96,7 @@ def is_stable_release(self) -> bool:
         else:
             raise ValueError("missing required context, require changelog or changes")
 
-        return dist in distro_info.UbuntuDistroInfo().supported()
+        di = distro_info.UbuntuDistroInfo()
+        stable = set(di.supported() + di.supported_esm()) - set([di.devel()])
+
+        return dist in stable