build: add Fedora 31 into CI / CD

Added build + test jobs in GitLab-CI and build + test + deploy jobs on
Travis-CI for Fedora 31.

Updated testing dependencies in the RPM spec to follow the new Python 2
package naming scheme that was introduced in Fedora 31: it uses
python2-' prefix rather then 'python-'.

Fedora 31 does not provide python2-gevent and python2-greenlet packages,
so they were pushed to https://packagecloud.io/packpack/backports
repository. This repository is enabled in our build image
(packpack/packpack:fedora-31) by default. Those dependencies are build-time,
so nothing was changed for a user. The source RPM packages were gathered
from https://rpms.remirepo.net/rpmphp/

.

Closes #4612

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>
(cherry picked from commit 9e09b07c)