It appears that strings are not auto-converted

This appears to work:
```common-lisp
(py4cl:python-eval "\"foo\"")
```
This appears not to work:
```common-lisp
(let ((my-str "foo"))
  (py4cl:python-eval my-str))
```

My expectation was that the library would handle converting `my-str` to a python string so that I can do things like pass it into functions, etc. Am I doing something wrong?