Make `HLSStreamWorker` reload the playlist in strict time intervals
instead of just queuing the next playlist reload.

Previously, playlist fetch time and segment processing time always added
to the overall reload time, resulting in an eventual shift of the
expected reload time, which could lead to an empty segment queue
in certain scenarios (live-edge=1 or Twitch low latency streaming).

When the reload time exceeds the expected reload interval, continue
with the shifted interval, which matches the old behavior.

Co-authored-by: Sebastian Meyer <mail@bastimeyer.de>

Fix import of `create_urllib3_context` method on urllib3 <2.0.0.
Only import from `urllib3.util.ssl_` module as a fallback.