replace `py_str` with `py_cmd` literal?

I think, semantically it makes more sense to use custom command literals instead of string literals for code in other languages, since I think it is important to distinguish code from plain strings. The default shell is already a different language within Julia, so I think it is reasonable to extend this concept to interop with other languages. This could also then be used by editors to customize syntax highlighting for Python code inside Julia, as already proposed in https://github.com/JuliaEditorSupport/julia-emacs/issues/132. The current string syntax could still be kept around, of course, and perhaps deprecated eventually. I would be curious to hear others' opinions on this.