feature: accept sentinel:// and redis+sentinel:// as scheme aliases

[sibson]

What & why

get_redis() only routed to the Sentinel branch when redbeat_redis_url
started with redis-sentinel. Celery's own native sentinel scheme is
sentinel:// and kombu's is redis+sentinel://, so a user who reuses their
broker DSN for redbeat_redis_url falls through to Redis.from_url and gets an
unhelpful failure.

Fix

Match all three scheme spellings:

elif (
    conf.redis_url.startswith(('redis-sentinel', 'sentinel://', 'redis+sentinel://'))
    and 'sentinels' in redis_options
):

Scope — please read

This is deliberately only a scheme match, not full celery-native sentinel
support. The and 'sentinels' in redis_options condition is unchanged, so this
helps only if redbeat_redis_options (or broker_transport_options) already
supplies an explicit sentinels list. A purely celery-native config —
master_name in broker_transport_options with hosts derived from a
;-joined sentinel:// URL — still falls through and still raises.

The CHANGES.txt entry and the new docs/config.rst paragraph both say so
explicitly, so nobody reads this as more than it is. If you'd rather this go
the whole way and derive sentinels/master_name from the URL, that's a
larger change and worth its own issue.

Test plan

• Added regression coverage in tests/test_scheduler.py for both new aliases.
• make test passes (91 tests).

Closes #199

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