admin/build-doc: always install python3-* for build deps
since we've dropped the support of python36-*, i.e. python packages
provided by EPEL7 before RHEL7/CentOS7 included python3. as before the
inclusion of python3 as supported python3, python packages are named
python36-*. and they don't provide python3-*. so we had to install
python36-* explicitly. now that we are able to use the python3-*
packages, we can just install python3-*.