Replace usage of total_seconds compat func with timedelta method
`total_seconds` and `tot_seconds` functions were apparently made for python 2.6 compat.  This removes them and replaces their usage with `timedelta.total_seconds` method.

There is one remaining use of this function in a C file.  I expect it will be easy to remove for someone who knows what they're doing.

See discussion https://github.com/pandas-dev/pandas/pull/17274#issuecomment-323363823

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
