install-deps.sh: install `python*-devel` for python*rpm-macros

install-deps.sh: install `python*-devel` for python*rpm-macros

in 087ea813, we installed '*rpm-macros' for the macros, so we can have
access to the latest python packaging related macros for preparing the
build dependencies.

but we could run into https://bugs.centos.org/view.php?id=16379, if
we already have an old version of python-devel installed. as the newer
version of python-rpm-macros conflicts with it.

it was a chicken-and-egg problem, as we don't know the exact name of
*rpm-macros packages. that's why we chose to install all of them. but
we have to upgrade the existing python-devel package to resolve the
conflict. but the since there is no python3-devel in RHEL7/CentOS7,
what they have is python36-devel. so we have to hardwire the
`%{python3_pkgversion}` to "36" even before we have access to this
macro, and upgrade the python36-devel package beforehand. but this
renders installing the rpm-macro package less useful -- we intend to
use the macro offered by the package to figure out "36".

as a workaround, we pretend that we know the "main" version of python3
in current RHEL/CentOS. and always install python36-devel for
python-rpm-macros. as the former requires the latter.

once all python3*-devel on all builders are upgraded, we will be safe
to install '*rpm-macros' again without installing python36-devel first.
by then, we could revert this change, or continue installing
python36-devel until the distro bumps up the "main" python version to 3.7

Fixes: https://tracker.ceph.com/issues/41603
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 0b2be2b6710695867197ee84e411bdf0a843a06a)

Conflicts:
	install-deps.sh
- VERSION_ID in luminous, MAJOR_VERSION in master