On windows setuptools install gpython.exe and gpython-script.py while
pip/distlib install gpython.exe with gpython-script sometimes embedded
into gpython.exe itself with argv[0] pointing to 'gpython' without .exe
suffix. This leads to gpython startup failure:

    (1.wenv) Z:\home\kirr\src\tools\go\pygo-win\pygolang>gpython
    Traceback (most recent call last):
      File "C:\Program Files\Python310\lib\runpy.py", line 196, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File "C:\Program Files\Python310\lib\runpy.py", line 86, in _run_code
        exec(code, run_globals)
      File "Z:\home\kirr\src\tools\go\pygo-win\1.wenv\Scripts\gpython.exe\__main__.py", line 7, in <module>
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 437, in main
        pymain(argv, init)
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 79, in pymain
        if not _is_buildout_script(exe):
      File "Z:\home\kirr\src\tools\go\pygo-win\pygolang\gpython\__init__.py", line 442, in _is_buildout_script
        with open(path, 'rb') as f:
    FileNotFoundError: [Errno 2] No such file or directory: 'Z:\\home\\kirr\\src\\tools\\go\\pygo-win\\1.wenv\\Scripts\\gpython'

    (1.wenv) Z:\home\kirr\src\tools\go\pygo-win\pygolang>dir ../1.wenv/scripts/gpython*

    Directory of Z:\home\kirr\src\tools\go\pygo-win\1.wenv\scripts

    26.04.2023     14:17       108,404  gpython.exe
           1 file                   108,404 bytes
           0 directories     88,508,866,560 bytes free

-> Adjust pymain to handle this case accordingly.