Pack RPM/DEB for obsolete distros

审查 PR #248 以及软件包测试和发布矩阵，然后查阅链接的关于构建 RPMs 的 wiki 草稿。检查 CentOS 7、Debian 9 和 Ubuntu 18.04（可能还包括 16.04）的依赖项可用性，包括 Python 3.6 的要求。完成的标准是：支持的 RPM/DEB 软件包能够针对选定的已停止支持的发行版，在其依赖项可用的情况下完成构建、测试和发布。

Follows up #248

In PR #248 we had introduced RPM and DEB packages for distros which have Python 3.7 or newer in its package managers by default. Extending package testing and publish matrices is a complicated task. Why is that?

First, the code works with Python 3.6 or newer, so if system doesn't have it one (including our CI) must set it up manually. Second, running code with Python 3.6 requires dataclasses backport package. And here we come to the packages issue.

It seems rather useless to provide a package for a distro if distribution repositories (or some external popular repos like EPEL) don't have its dependencies. And the case is they mostly don't.

RPM with dataclasses backport is available for EPEL 8 only. It is not so hard to rebuild (see https://github.com/tarantool/tarantool-python/wiki/Building-RPMs draft), but we will need to provide and maintain external packages in Tarantool repo. I also haven't seen any DEBs for dataclasses.

dataclasses is not the only one troublesome package. For example, CentOS 7 (including EPEL 7) does not provide any pandas RPM for Python 3. Rebuilding pandas RPM from EPEL 8 on EPEL 7 is rather challenging task. We also shouldn't forget that pandas has many dependencies and it is possible that we would need to rebuild them to.

Distros planed in this ticket:

• centos:7
• debian:9
• ubuntu:18.04 (and possibly 16.04)

centos:8 is EOL so we don't plan to provide a package for it.