gpython: Include gevent version into banner

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

gpython/__init__.py

@@ -150,6 +150,7 @@ def main():
                 '\n\n\t%s\n\nsys.modules:\n\n\t%s' % (bad, sysmodv))
 
     # make gevent pre-available & stdlib patched
+    import gevent
     from gevent import monkey
     # XXX workaround for gevent vs pypy2 crash.
     # XXX remove when gevent-1.4.1 is relased (https://github.com/gevent/gevent/pull/1357).
@@ -182,7 +183,7 @@ def main():
         exe = exe + '.exe'
 
     sys.executable  = exe
-    sys.version    += (' [GPython %s]' % golang.__version__)
+    sys.version    += (' [GPython %s] [gevent %s]' % (golang.__version__, gevent.__version__))
 
     # tail to pymain
     pymain(sys.argv[1:])

gpython/gpython_test.py

@@ -91,9 +91,11 @@ def test_gevent_activated():
 @gpython_only
 def test_executable():
     # sys.executable must point to gpython and we must be able to execute it.
+    import gevent
     assert 'gpython' in sys.executable
     out = pyout(['-c', 'import sys; print(sys.version)'])
     assert ('[GPython %s]' % golang.__version__) in str(out)
+    assert ('[gevent %s]'  % gevent.__version__) in str(out)
 
 # verify pymain.
 #