To match python behaviour:

    $ python >/dev/null
    Python 2.7.18 (default, Apr 20 2020, 20:30:41)
    [GCC 9.3.0] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> 1+1			# NOTE the prompt (printed to stderr)

    $ python 2>/dev/null
    >>> 1+1			# NOTE the prompt (printed to stdout)

Tests pending.

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!15