PEP 394 expects that Python 3 installations provide a `python3` command,
but does not require a `python` command. Some distributions (including
Debian) are no longer providing a `python` command, but do provide
`python3`.
In this change, the `python3` interpreter is invoked via the existing
`#!` line in the `testspec.py` script.
Signed-off-by: Matthew Brecknell <Matthew.Brecknell@data61.csiro.au>