Fix exception cause in config.py (#488)

Author: cool-RR

src/pydocstyle/config.py

@@ -463,7 +463,7 @@ def _expand_error_codes(code_parts):
                         'known errors: %s', part)
                 expanded_codes.update(codes_to_add)
         except TypeError as e:
-            raise IllegalConfiguration(e)
+            raise IllegalConfiguration(e) from e
 
         return expanded_codes