Croniter

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- Enforce validation for month=1. Before this release we used to support month=0 and it was silently glided to month=1 to support having both day in month in 4th field when it came to have 6fields cron forms (second repeat). It will now raises a CroniterBadDateError. See https://github.com/kiorky/croniter/issues/6
[kiorky]

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- restore py2 [kiorky]

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- better type checks [kiorky]

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- Fix ZeroDivisionError with ``* * R/0 * *``
[cuu508]

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- Add support for hashed/random/keyword expressions
Ryan Finnie (rfinnie)
- Review support support for hashed/random/keyword expression and add expanders reactor
[ kiorky ]

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- fix bug: bad case:``0 6 30 3 *``
[zed2015(zhangchi)]
- Add support for ``L`` in the day_of_week component. This enable expressions like ``* * * * L4``, which means last Thursday of the month. This resolves 159.
[Kintyre]
- Create ``CroniterUnsupportedSyntaxError`` exception for situations where CRON syntax may be valid but some combinations of features is not supported.
Currently, this is used when the ``day_of_week`` component has a combination of literal values and nth/last syntax at the same time.
For example, ``0 0 * * 1,L6`` or ``0 0 * * 15,sat1`` will both raise this exception because of mixing literal days of the week with nth-weekday or last-weekday syntax.
This *may* impact existing cron expressions in prior releases, because ``0 0 * * 15,sat1`` was previously allowed but incorrectly handled.
[Kintyre]

- Update ``croniter_range()`` to allow an alternate ``croniter`` class to be used. Helpful when using a custom class derived from croniter.
[Kintyre]