Migrate the python2.7 to python3.7

[zymap]

Motivation

PIP-56 proposed we should upgrade our python version in the Docker image, we did that in the PR #7963.
But I still find some python2.7 usage in the pulsar repository:

function module:
https://github.com/apache/pulsar/blob/master/pulsar-functions/scripts/python/generate.sh

cpp module:
https://github.com/apache/pulsar/blob/master/pulsar-client-cpp/CMakeLists.txt#L175
https://github.com/apache/pulsar/blob/master/pulsar-client-cpp/python/CMakeLists.txt#L40https://github.com/apache/pulsar/blob/master/pulsar-client-cpp/python/pkg/osx/vagrant-build.sh#L38

Should we remove them as well?

[BewareMyPower]

The C++ related build and test scripts rely on the docker image apachepulsar/pulsar-build:ubuntu-16.04.

Dockerfiles under https://github.com/apache/pulsar/blob/master/pulsar-client-cpp/docker directory also need changes.

[BewareMyPower]

It's already done by PIP-155.