Allow auto conversion of Python examples on Windows

3to2 cannot be called as an executable on Windows, but it can be called
as a script. Assume that the 3to2 script is installed in
PYTHONROOT\Scripts, which is the default for installation by pip.
`os.path.dirname(which(env['python_cmd']))` might be better here, since we don't know whether an absolute path was given for `python_cmd`. Ditto for the other instance of this.