I had a code that would freeze, and after debugging I realized that a bug in my code caused `shlex.split(None)` to be called, which puts Python in an infinite loop.

We probably want to do an `if not isinstance(s, ...): raise TypeError('Bla bla')` at the start of the `shlex` function, or maybe at `shlex.__init__`, or maybe both.

What bothers me though, is that it seems that shlex can accept both a string and a stream, so we should allow both of these and not others. Now, I don't know how to check whether it's a proper stream. Would `isinstance(s, io.IOBase)` be good enough? Will it catch all of the streams that people might use in `shlex.split`?

(Also, I recently proposed something related in python-ideas: https://groups.google.com/forum/#!searchin/python-ideas/stream%7Csort:relevance/python-ideas/uYlnnH52VEA/PSHkQRtaBwAJ )