Typo in interval handling

[DirkReiners]

I'm getting exception when trying to set an interval in the Recurrence. I think the problem is line 447 in recurrence.py should be

if interval and (not isinstance(interval, int) or interval < 1):

instead of

if interval and (isinstance(interval, int) or interval < 1):

That seems to fix it for me, at least. ;)

[kuzmoyev]

@DirkReiners Thank you for the issue submission. It is a typo. Fixed in dev.