API: restore timedelta compability for Timedelta components?
With the new `Timedelta` type (see https://github.com/pydata/pandas/pull/8184), we introduced behavior that breaks compatibility with the superclass `timedelta`: `td.seconds` refers to seconds since the last minute, rather than seconds since the last day (as in the superclass).

I think breaking compatibility with the superclass here was a mistake. User code clearly _does_ rely on the old behavior, e.g., as manifested in bugs for exporting datetime columns to Excel: #9139. Moreover, it's likely to remain an issue causing silent bugs, because external libraries are going to continue to use `isinstance` checks to verify that something is a `datetime`/`timedelta` object. We chose to subclass `timedelta` here, so we really are stuck with full API compatibility.

So I think we should make this change in 0.16. It's a breaking change, but 0.15 was already a breaking change in this respect, and in any case I think we should give ourselves some allowance for API changes for new types.

CC @jorisvandenbossche @jreback 
