chore: Use python 3.6.13-buster for owlbot post-processor (#1022)

Author: parthea

docker/owlbot/python/Dockerfile

@@ -16,7 +16,7 @@
 
 # build from the root of this repo:
 # docker build -t gcr.io/cloud-devrel-kokoro-resources/owlbot-nodejs -f docker/owlbot/nodejs/Dockerfile .
-FROM python:3.9.4-buster
+FROM python:3.6.13-buster
 
 WORKDIR /