Croniter

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- fix ``get_next`` while preserving the fix of ``get_prev`` in 7661c2aaa
[Avikam Agur <avikampagaya-inv.com>]

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- Don't count previous minute if now is dynamic
If the code is triggered from 5-asterisk based cron
``get_prev`` based on ``datetime.now()`` is expected to return
current cron iteration and not previous execution.
[Igor Khrol <igor.khroltoptal.com>]

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- More DST fixes
[Kevin Rose <kbrosegithub>]

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- fix 87: backward dst changes
[kiorky]

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- Add is valid method, refactor errors
[otherpirate, Mauro Murari <mauro_murarihotmail.com>]

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- DOW occurrence sharp style support.
[kiorky, Kengo Seki <sekiknapache.org>]