Move python version getter to utils

There was a circular dependency between `py3compat.py` and `utils.py`

I solved setting the imports from `py3compat` in the functions

https://github.com/slok/circus/blob/py-version-util/circus/util.py#L239
https://github.com/slok/circus/blob/py-version-util/circus/util.py#L264

I don't think that is the best approach :/