Removed all 2.6 herritage

Originally: Changed seconds calculation for `timedelta.total_seconds()`
A small change in differential_rotation

But now it includes few other changes, not just `total_seconds()` but the newer formatting to all the code, except to 3 files:

- `setup.py`
- `counter.py` line 118 - I haven't found a work around the `__mod__`
- `Imageanimator` line 33 - another needed work around for `valfmt`.

Minor: I think the string formatter is the default one, so just using ``{}`` should do the same.