allura/scm_config/git-http has a Dockerfile with an older LTS version of Ubuntu that support python2 but latest LTS version of Ubuntu has removed python2 related packages.
So we need to upgrade to python3 in order to update the latest LTS version to keep it in sync with the base Dockerfile which was upgraded in here https://forge-allura.apache.org/p/allura/tickets/8513/
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