`move_message()` should use `renameat2` on non-ancient Linux

Now that CentOS 7 is history, we could use an atomic file swap: https://github.com/jordemort/python-renameat2

Also need to package the dependency in the COPR repo.

Note to self: [atomicswap](https://github.com/nickovs/atomicswap/tree/master/src/atomicswap) is somewhat related, supports also Windows and MacOS but only provides the "swap" part, not the atomic rename (and is Python 3.7+).
