COMPAT: periods needs coercion to np.int64 in _generate_regular_range
https://github.com/pydata/pandas/blob/master/pandas/tseries/index.py#L1768

a simple `np.int64(periods)` will suffice

(and tests of course)

This needs to guaranteed to be int64 as a result (e.g. if this is in32 it will have an internal overflow which will make it have a valid but weird result)
