rpm: use Python 3 in testing on RPM packaging

Fix package names with Python modules on Centos 7, OpenSUSE, Fedora.
All supported RPM-based distributives (Fedora 33, Fedora 32, OpenSUSE 15.2,
OpenSUSE 15.1, CentOS 8) contains python3, python3-six and python3-gevent
packages except CentOS 7 and CentOS 6.
We don't run tests when make packages for CentOS 6. For CentOS 7 we create
our own RPM packages for Python3 dependencies.

Patch unblocks regression testing on Fedora 33.

Part of #5652
Closes tarantool/tarantool-qa#17