Support running test suite using python3

[David Bremner]

On systems where the default python is python2 (i.e. most of them),
there's not convenient way to try out the test suite with python3.
This series aims to make that easier by making

% PYTHON=python3 ./configure

propagate something useful to the test suite.

The last patch is not strictly needed, but it helped me figure out
what was wrong when the python3 version of sphinx was not installed.