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RHEL6 builders fail to build because python3 and python3-pip rpm
packages are not available there (and Python 3 is installed from
source).

Create builder-* includes per RHEL/CentOS Stream release and integrate
the python3, python3-pip and python3-devel packages there.

For RHEL <7.7, create disabled-by-default yum repos for RHEL 7.7 that
can be used to pull in newer packages on-demand. Install python3-devel
everywhere, because if we ever move to native tool builds for those RHEL
versions some packages installed in the pipeline prepare job might need
it during installation from source for non-x86 arches. Install both
Python 2 and Python 3 from the RHEL 7.7 repos, as older python-devel
packages conflict with python3-devel from RHEL 7.7.
Signed-off-by: Michael Hofmann <mhofmann@redhat.com>