pymethod don't work with string

```common-lisp
(py4cl2:pycall "len" #(1 2 3 4))   ;; work
(py4cl2:pymethod #(1 2 3) 'append 10) ;; work
(py4cl2:pymethod #(1 2 3) "append" 10) ;; error
```
I think it should be support string. because some python library use camelCase. 