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`.

I would argue against changing this file, partly because I think we remove it when we merge astropy helpers and secondly because I don't actually want to physically prevent people installing sunpy under 2.6, just in case.