Strip possibly present trailing / before adding one

Without that users must have trailing / removed, but nobody checks for that etc.
So now we have "raw//" etc, e.g. at
https://bids-specification.readthedocs.io/en/latest/derivatives/common-data-types.html#example-use-of-a-descriptionstsv-file

With this change it would make it easier on user and guaranteed to render consistently correctly

tools/examplecode/example.py

@@ -69,7 +69,7 @@ def _add_dictionary(
 
         # We are dealing with a directory
         else:
-            self._tree.append(f"{prefix}{connector} {entry}{os.sep}")
+            self._tree.append(f"{prefix}{connector} {entry.rstrip(os.sep)}{os.sep}")
             prefix += (
                 self.PIPE_PREFIX if index != entries_count - 1 else self.SPACE_PREFIX
             )