gpython: Fix `gpython -X gpython.runtime=threads` to spawn subinterpreters with threads runtime by default

Previously it was not the case and gpython with default being gevent
runtime was spawned even if parent gpython was instructed to use threads runtime:

    (z-dev) kirr@deca:~/src/tools/go/pygolang$ gpython -X gpython.runtime=threads
    Python 2.7.18 (default, Apr 28 2021, 17:39:59)
    [GCC 10.2.1 20210110] [GPython 0.1] [threads] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>> import sys
    >>> sys.version
    '2.7.18 (default, Apr 28 2021, 17:39:59) \n[GCC 10.2.1 20210110] [GPython 0.1] [threads]'   <-- NOTE threads
    >>> import subprocess
    subprocess.call(sys.executable)
    Python 2.7.18 (default, Apr 28 2021, 17:39:59)
    [GCC 10.2.1 20210110] [GPython 0.1] [gevent 21.1.2] on linux2                               <-- NOTE gevent
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>>

/proposed-for-review-on nexedi/pygolang!25