Gevent is major component gpython builds on - it deserves to be included
into version and is handy to know if one hits a bug with gpython - to
see in which particular runtime environment the bug was hit.

Before:

    $ gpython
    Python 3.7.3 (default, Dec 20 2019, 18:57:59)
    [GCC 8.3.0] [GPython 0.0.6.post2] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>>

After:

    $ gpython
    Python 3.7.3 (default, Dec 20 2019, 18:57:59)
    [GCC 8.3.0] [GPython 0.0.6.post2] [gevent 20.5.1] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    (InteractiveConsole)
    >>>

/cc @gabriel, @jerome
/proposed-for-review-on: nexedi/pygolang!4