Fix ruff alert (currently) not caught by pre-commit

TRY002 Create your own exception

I changed `Exception` to `RuntimeError`, but the exception cannot be thrown
any way. Any file can be read with encoding "iso-8859-1", UnicodeDecodeError
will never be raised with this encoding. Add "no cover" to avoid Codecov
warning.

codespell_lib/_codespell.py

@@ -269,8 +269,9 @@ def open_with_internal(self, filename: str) -> Tuple[List[str], str]:
                 else:
                     break
         else:
+            # reading with encoding "iso-8859-1" cannot fail with UnicodeDecodeError
             msg = "Unknown encoding"
-            raise Exception(msg)
+            raise RuntimeError(msg)  # pragma: no cover
 
         return lines, encoding