Closes #8943 COMPAT: periods needs coercion to np.int64 for v0.16.0

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.

(bug fixes in v0.16.0)