We were not (and probably wont be ) using any worthwhile `requests`
features (beside `raise_for_status()`) and the `timeout` session
parameter propagation vs adapter plugging "thing" in requests just
annoys me deeply (not that kind of "... Human (TM)")
Remove dependency to `webvtt-py` which was both too much and not enough
for our use case.
Implement a basic WebVTT to SRT converter according to ArteTV's usage of
WebVTT features.
