ceph.spec.in: use python_provide macro

ceph.spec.in: use python_provide macro

our python3 bindings are now named `python3-<modname>` after python3 is
now maintained by RHEL/CentOS instead EPEL. to help the users using
`python36-<modname>`, we should "Provide" `python36-<modname>`.

the `python_provide` macro is offered by `python-rpm-macros` package,
which is in turn required by python*-devel. and we do install
`python36-devel` in install-deps.sh, and install `python3-devel` in
ceph-*build/build/setup_rpm

see also
https://docs.fedoraproject.org/en-US/packaging-guidelines/Python/#_provides

Signed-off-by: Kefu Chai <kchai@redhat.com>