Adds TUI mode to OpenBSD.

Issue: Entering TUI mode currently only checks for Linux or Darwin, ignoring BSD systems despite being compatible.

Change: This commit allows OpenBSD to use cobib's TUI mode.

Additional:

It should also be possible to enable TUI mode on other BSDs by extending the relevant checks to contain:

    if not args.command:
        if platform.system() not in ("Linux", "Darwin", "OpenBSD", "FreeBSD", "NetBSD"):
            LOGGER.error("Only ... systems are supported by coBib's TUI!")
            return

I haven't included those changes in this commit as I don't have access to other versions of BSD to check such a change works.

Proof: