Function find_interpreter 

pub fn find_interpreter() -> Result<PathBuf, Error>

Attempts to locate a python interpreter.

Locations are checked in the order listed:

1. If PYO3_PYTHON is set, this interpreter is used.
2. If in a virtualenv, that environment’s interpreter is used.
3. python, if this is functional a Python 3.x interpreter
4. python3, as above