This function is supposed to be called with script names and a directory
to store the scripts, as described in
https://github.com/buildout/buildout/blob/2.7.1/src/zc/buildout/easy_install.txt#L478

Passing an absolute path for script name and None as directory was OK on
python <= 3.5, because:
  os.path.join(None, '/absolute/path')
returns '/absolute/path', but on python >= 3.6 this is no longer the
case and failed with:

    Traceback (most recent call last):
      File "slapos.buildout/src/zc/buildout/buildout.py", line 2240, in main
        getattr(buildout, command)(args)
      File "slapos.buildout/src/zc/buildout/buildout.py", line 643, in install
        self._install_parts(install_args)
      File "slapos.buildout/src/zc/buildout/buildout.py", line 658, in _install_parts
        self._setup_directories()
      File "slapos.rebootstrap/slapos/rebootstrap/__init__.py", line 37, in wrapper
        return getattr(self, attr)(*args, **kw)
      File "slapos.rebootstrap/slapos/rebootstrap/__init__.py", line 72, in _setup_directories
        setup_script(new_bin, self.wanted_python)
      File "slapos.rebootstrap/slapos/rebootstrap/bootstrap.py", line 31, in setup_script
        python,
      File "slapos.buildout/src/zc/buildout/easy_install.py", line 1313, in scripts
        sname = os.path.join(dest, sname)
      File "python3/lib/python3.6/posixpath.py", line 80, in join
        a = os.fspath(a)
    TypeError: expected str, bytes or os.PathLike object, not NoneType

Change the parameters to respect the original signature.