Closes #8943 COMPAT: periods needs coercion to np.int64

In _generate_regular_range, we explicitly cast `periods` to np.int64 to prevent
overflow in the function tseries.index._generate_regular_range.
Note: we don't bother casting it for the function call generate_range since
there's no danger of overflow in that function.
Merge pull request #9078 from Garrett-R/fix_8943

Closes #8943:  COMPAT: periods needs coercion to np.int64
overflow bug in multi-index when checking for duplicates
Merge pull request #9101 from behzadnouri/mi-dups

overflow bug in multi-index when checking for duplicates
BUG: Bug in DatetimeIndex iteration, related to (GH8890), fixed in (GH9100)
Merge pull request #9104 from jreback/dti_iteration

BUG: Bug in DatetimeIndex iteration, related to (GH8890), fixed in (GH9100)
unstack with nan in the index