Drop 3.3, 3.4 and use single version docker images for all python tests (#7396)
* Create a new docker image for python 3.6
The previous one cannot build, because python3.6 was removed from
stretch.
* Drop support for 3.3 and 3.4
Also update all docker file to be single version
diff --git a/kokoro/linux/python35_cpp/build.sh b/kokoro/linux/python35_cpp/build.sh
index 2a79246..4d0dbd4 100755
--- a/kokoro/linux/python35_cpp/build.sh
+++ b/kokoro/linux/python35_cpp/build.sh
@@ -11,7 +11,7 @@
cd $(dirname $0)/../../..
export DOCKERHUB_ORGANIZATION=protobuftesting
-export DOCKERFILE_DIR=kokoro/linux/dockerfile/test/python_stretch
+export DOCKERFILE_DIR=kokoro/linux/dockerfile/test/python35
export DOCKER_RUN_SCRIPT=kokoro/linux/pull_request_in_docker.sh
export OUTPUT_DIR=testoutput
export TEST_SET="python35_cpp"
