The boolean flags `--extm3u` and `--no-extm3u` are replaced with a string option `--output=m3u|m3u8`.
This reduces the amount of options and allows to evolve the CLI to support more playlist output formats in the future (e.g. JSON) without polluting the CLI at that point.
This assertion was silently a no-op for years, and it apparently fails.
So since we were not even testing it before, it is at least no worse to
just remove it.
AFAICT, `mock.has_calls` *never* existed. It only started emitting a
warning recently. And for some reason I only see this crop up on
Windows? Truly mysterious.