fix #99: implement commented string_quote_removal

[WillForan]

No description provided.

[idank]

Thanks, can you make this work in 3.7 as well?

[WillForan]

I worried the walrus operator was suspicious, albeit more concise. hopefully removing it does the trick for py3.7

[idank]

There seems to be more to this function than just single quotes: https://github.com/bminor/bash/blob/master/subst.c#L11892

Like double quotes, escape characters. Can you maybe leave a todo if you're not going to add that part?