fix: update default python version

python.json

@@ -5,7 +5,7 @@
   ],
   "dependencyDashboardHeader": "Uses https://github.com/bettermarks/renovate-config#python  \nPR that have automerge enabled [might be approved automatically](https://github.com/bettermarks/approve-dependency-pr#readme)",
   "constraints": {
-    "python": "3.6"
+    "python": "==3.8"
   },
   "pip-compile": {
     "fileMatch": ["(^|/)requirements(-.*)?\\.in$"]