cpp: [:-1] causes: Advanced Slicing not supported
It seems this syntax is producing an exception only on pycpp, and only on py38
```py
GIT_PAGER=cat git grep '\[:-1\]'
py2many/scope.py:        return ScopeList(self[:-1])
pycpp/plugins.py:        return "\n".join(buf[:-1]) + "\nstd::cout << std::endl;"
pynim/plugins.py:        args = ", ".join(args[:-1])
```
occurs at https://github.com/adsharma/py2many/pull/421/checks?check_run_id=3052585490