COPR repo: introduce "avocado-latest-release"

[clebergnu]

There's one gap in the current offers of Avocado packages, which is allowing users to install the latest Avocado release (not the latest development version).

This was previously covered by the use of the "latest" stream in distributions that provided the "modular" concept (Fedoras, EPEL, etc).

The definition of done here is to have a repo that users can enable and use like this:

dnf copr enable @avocado/avocado-latest-release
dnf install python3-avocado*