[python][test] Use python3 in dune_python_add_test

This was hard-coded to python before but a better default might be python3 since we require Python 3 (According to https://peps.python.org/pep-0394/, python doesn't even have to exist and if it exists may point to either Python 2 or 3).

This also fixes a problem that came up in !1222 (merged) when trying to use the Dune bindings without a virtual env on a system where python points to python2.