Function call issue for parsing sentinel_fallback (backport #1405)

[mergify]

Hi,
The _parse_sentinel was a class function and when it get called by

sentinel_hosts = [
            _parse_sentinel(fallback)
            for fallback in kwargs.pop('sentinel_fallback', [])
        ]

It made errors because it doesn't pass cls to it and it can't receive the sentinel.
So I made it static and tested it and its OK now.

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This is an automatic backport of pull request #1405 done by Mergify.