docs not building due to `execfile not defined`

It looks like there is a line in the docs that prevents the docs from building under python3

https://github.com/pyatom/pyatom/blob/a170717408d46f5d872585a877ffd8ee09a1a052/doc/conf.py#L72

compare to what's going on in `setup.py`

https://github.com/pyatom/pyatom/blob/a170717408d46f5d872585a877ffd8ee09a1a052/setup.py#L22

Gonna fix this in a PR in a second.