set enabled Python bindings uniformly on RHEL/CentOS

set enabled Python bindings uniformly on RHEL/CentOS

0af0dc91894f2598f0e19e3cd2c1b34925c9cd61 introduced a workaround for Python
packages changing names. This was relevant in the era of CentOS 6. However, now
Graphviz no longer supports CentOS 6 and furthermore CentOS 7 has converged on
the same naming as CentOS 8. As a result, the Graphviz packaging work flow was
actually *disabling* Python 3 bindings and enabling Python 2 bindings on CentOS
7.

This change unifies which Python bindings are enabled across RHEL and CentOS
versions. Though an upcoming change will remove support for the Python 2
bindings altogether. Related to #1992.