Fix the type hint for Field.converter

`Field.converter` is called in `Field.validateValue()` on the result of
`Field.extractValue()`, which will return either a string,
JSON-serializable objects, or None, but not bytes.

src/klein/_form.py

@@ -89,7 +89,7 @@ class Field:
     @ivar converter: The converter.
     """
 
-    converter: Callable[[AnyStr], Any]
+    converter: Callable[[str], Any]
     formInputType: str
     pythonArgumentName: Optional[str] = None
     formFieldName: Optional[str] = None