There was an assert because sys.path[0] is setup as realpath (real, not
relative), but if exe was given as relative path it was raising like
below:

    gpython/gpython_test.py::test_pymain_run_via_relpath GPython 0.0.7.post1 [gevent 20.9.0] / CPython 3.8.6
    Traceback (most recent call last):
      File "./__init__.py", line 482, in <module>
        main()
      File "./__init__.py", line 366, in main
        pymain(argv, init)
      File "./__init__.py", line 82, in pymain
        raise RuntimeError('pymain: internal error: sys.path[0] was not set by underlying python to dirname(exe):'
    RuntimeError: pymain: internal error: sys.path[0] was not set by underlying python to dirname(exe):

            exe:    ./__init__.py
            sys.path[0]:    /home/kirr/src/tools/go/pygolang-master/gpython
    FAILED

-> Fix it by setting up exe as realpath.