[fix][cpp-client] Add python3 to rpm build Dockerfile

[yaalsn]

Motivation

Since PIP-155, all the python scripts in pulsar had been upgraded to python3, so we need to install python3 into Docker image apachepulsar/pulsar-build:centos-7 to run these scripts.

Verifying this change

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

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More

This PR should be cherry-picked to 2.11 branch and rebuild a image via running pulsar-client-cpp/pkg/rpm/build-docker-image.sh and republish to Dockerhub.