Unreached/Unreachable(?) paths in tslib
in `tslib._Timedelta.__richcmp__` there is a path that is not reached in tests that I think may not be [reachable](https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslib.pyx#L2625), seeking second opinions.  If there are corner cases where this can be reached, they should be added to the tests.

```
        if isinstance(other, _Timedelta):
            if isinstance(other, _NaT):
                return _cmp_nat_dt(other, self, _reverse_ops[op])
```