pygolang: Accompany python-interpreter with gpython-interpreter to run gpython instead of std python

With python-interpreter staying the default for pyprog, but possible to adjust.

We will need gpython-interpreter for ERP5 to use bstr+ustr as builting string types.

Remove [gpython] script in favour of gpython-interpreter since both provide the
same thing but via slightly different ways internally ([gpython] was generating
bin/gpython via setuptools and [gpython-interpreter] does it by hand with
user-visible result staying the same).

/cc @kazuhiko, @jerome