[MM3-users] Can't import from Mailman 2.1

I set up a new Mailman site using 3.3.10, and copied over my files from my old mailman site (using 2.1.34). I created a new domain, and a new list, and tried to import the settings from the old pickle file using mailman import21 pt-test@browncroftna.org /var/tmp/var.lib.mailman/lists/pt-test/config.pck (in the virtual environment shell) but I get the following error

File "/opt/mailman/venv/lib/python3.13/site-packages/mailman/utilities/importer.py", line 512, in import_config_pck
    default_value = getUtility(ITemplateLoader).get(newvar, mlist)
  File "/opt/mailman/venv/lib/python3.13/site-packages/mailman/model/template.py", line 211, in get
    path, fp = find(default_uri, mlist, code)
               ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/mailman/venv/lib/python3.13/site-packages/mailman/utilities/i18n.py", line 157, in find
    for search_path in raw_search_order:
                       ^^^^^^^^^^^^^^^^
  File "/opt/mailman/venv/lib/python3.13/site-packages/mailman/utilities/i18n.py", line 117, in search
    templates_dir = str(resources.enter_context(
                        ~~~~~~~~~~~~~~~~~~~~~~~^
        files('mailman').joinpath('templates')))
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/contextlib.py", line 528, in enter_context
    raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does "
                    f"not support the context manager protocol") from None
TypeError: 'pathlib._local.PosixPath' object does not support the context manager protocol

All I can find on Google is a suggestion that I have a non-empty bounce_info, but dumpdb shows it as empty.