Improve rpm packages build

[pgiraud]

The process to build the rpm packages and its documentation are not up-to-date.
For example, in the main setup.py the version is automatically guessed from the tag using use_scm_version. The howto release a new version in CONTRIBUTING.md has been updated accordingly but the rpm build process hasn't.

In the CI, the rpm build is actually currently creating a package for version 0.15.3 because it's hard written.

For the packages we want to distribute on yum.dalibo.org, we could probably let the CI build the packages for us and then download the generated artifacts to push them on our repository.

[dlax]

In the CI, the rpm build is actually currently creating a package for version 0.15.3 because it's hard written.

The same is true for debian/changelog which currently needs manual update. However, it might be worth trying to guess the version from `python setup.py --version` or `git describe` and insert the value in .spec file (or update d/changelog). If that's too much work w.r.t. benefit, we can also come back to setting the version value in those three files by hand and remove setuptools-scm usage.

For the packages we want to distribute on yum.dalibo.org, we could probably let the CI build the packages for us and then download the generated artifacts to push them on our repository.

Agreed, that'd be very nice.