Add python input variable

This MR adds a python spec:inputs variable, defaulting to "python", to allow users to customise which interpreter should be used.

This is most useful on systems that provide python3, but don't provide a default unversioned python (e.g. Debian and Red Hat).

The default is set to "python" to work with the majority of distributions, including on Windows, which never provides python3 (AFAIK).