Support hermetically built python interperters

[dmadisetti]

Subpar fails when py_runtime is provided a label for interpreter location, see: #98 (comment)

@thundergolfer brings up the point that for nix builds, or other hermetic python builds, subpar will not work. Ideally subpar should bundle the interpreter binary into the par for these instances.

I see the subpar is not longer being actively maintained, so no big deal. I think it's still important to have this issue open in case other people run into the same issue.