Small refactor and isort+black formatting

src/pyrmv/errors/ps_errors.py

@@ -6,8 +6,9 @@ class PsIncorrectParamError(Exception):
     forward/backward from the end/beginning of the
     connection.
     """
+
     def __init__(self):
         super().__init__(self.__doc__)
 
     def __str__(self):
-        return self.__doc__
+        return self.__doc__