Because else sys.modules['__main__'] points to the module of gpython
itself, not user program. This difference can be important when e.g.
user defines classes in main module and tries to pickle them. Such
pickling will fail if gpython is not careful to run user's code in its
own main.

Without this patch added test fails as

      File "check_main.py", line 51, in <module>
        main()
      File "check_main.py", line 39, in main
        assert hasattr(mainmod, 'MyUniqueClassXYZ'),        dir(mainmod)
    AssertionError: ['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'main']

The problem was discovered while trying to run test program from
https://github.com/zopefoundation/ZEO/issues/166 :

    $ gpython zopenrace-zeo.py
    Traceback (most recent call last):
      File "/home/kirr/tmp/trashme/Z/py2.venv/bin/gpython", line 8, in <module>
        sys.exit(main())
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 395, in main
        pymain(argv, init)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 266, in pymain
        run()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 172, in run
        _execfile(filepath, g)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/gpython/__init__.py", line 275, in _execfile
        six.exec_(code, globals, locals)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/six.py", line 719, in exec_
        exec("""exec _code_ in _globs_, _locs_""")
      File "<string>", line 1, in <module>
      File "zopenrace-zeo.py", line 205, in <module>
        main()
      File "zopenrace-zeo.py", line 190, in main
        init()
      File "zopenrace-zeo.py", line 126, in init
        transaction.commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_manager.py", line 257, in commit
        return self.manager.commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_manager.py", line 134, in commit
        return self.get().commit()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 282, in commit
        reraise(t, v, tb)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 273, in commit
        self._commitResources()
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 456, in _commitResources
        reraise(t, v, tb)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/transaction/_transaction.py", line 430, in _commitResources
        rm.commit(self)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 497, in commit
        self._commit(transaction)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 546, in _commit
        self._store_objects(ObjectWriter(obj), transaction)
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/Connection.py", line 578, in _store_objects
        p = writer.serialize(obj)  # This calls __getstate__ of obj
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/serialize.py", line 430, in serialize
        return self._dump(meta, obj.__getstate__())
      File "/home/kirr/tmp/trashme/Z/py2.venv/lib/python2.7/site-packages/ZODB/serialize.py", line 439, in _dump
        self._p.dump(state)
    PicklingError: Can't pickle <class '__main__.PInt'>: attribute lookup __main__.PInt failed

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