Export Sentinel, and SSL like other classes

[terencehonles]

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• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
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Description of change

Now that Sentinel needs to be used specifically instead of via Redis, this change exports the class at the top level like the Redis client is, and also allows the shorthand of ssl=True to set the connection class to the SentinelManagedSSLConnection.

[terencehonles]

Now that Sentinel needs to be used specifically instead of via Redis, this change exports the class at the top level like the Redis client is

The export may not be the issue I'm actually trying to fix. It looks like the project I'm contributing to does import Sentinel from the full path. The ssl change is complimentary to #1419, and I can back out the re-export if needed. I'm going to continue trying to track down why the code I'm looking at is breaking with the more recent changes.

[chayim]

Thanks @terencehonles I fully agree, we should be exposing this. I hope it fixes your bug. Thank you so much for the contribution!