📌(backend) pin pylint to 3.x to resolve compatibility conflict

Restrict pylint version to 3.x in renovate configuration because
pylint-django 2.6.1 requires pylint<4, preventing automatic upgrades
to pylint 4.x that would create unresolvable dependency conflicts
until pylint-django releases compatible version.

renovate.json

@@ -9,6 +9,12 @@
       "matchManagers": ["pep621"],
       "matchPackageNames": ["redis"]
     },
+    {
+      "groupName": "allowed pylint versions",
+      "matchManagers": ["pep621"],
+      "matchPackageNames": ["pylint"],
+      "allowedVersions": "<4.0.0"
+    },
     {
       "enabled": false,
       "groupName": "ignored js dependencies",