`UnknownLocalError` on for Occitan (`oc`)

[cfm]

Description

Despite the apparent resolution of python-babel/babel#553 in Babel 2.6.0, Occitan (oc) raises babel.core.UnknownLocaleError: unknown locale 'oc'.

Steps to Reproduce

make dev on securedrop-i18n@i18n.

Expected Behavior

The development environment starts without error.

Actual Behavior

Traceback (most recent call last):
  File "./manage.py", line 452, in <module>
    _run_from_commandline()
  File "./manage.py", line 442, in _run_from_commandline
    rc = args.func(args)
  File "./manage.py", line 99, in reset
    with context or app_context():
  File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/home/user/securedrop/securedrop/management/__init__.py", line 11, in app_context
    with journalist_app.create_app(config).app_context():
  File "/home/user/securedrop/securedrop/journalist_app/__init__.py", line 92, in create_app
    i18n.configure(config, app)
  File "/home/user/securedrop/securedrop/i18n.py", line 200, in configure
    usable_locales = validate_locale_configuration(config, babel)
  File "/home/user/securedrop/securedrop/i18n.py", line 138, in validate_locale_configuration
    available = set(babel.list_translations())
  File "/opt/venvs/securedrop-app-code/lib/python3.8/site-packages/flask_babel/__init__.py", line 155, in list_translations
    result.append(Locale.parse(folder))
  File "/opt/venvs/securedrop-app-code/lib/python3.8/site-packages/babel/core.py", line 331, in parse
    raise UnknownLocaleError(input_id)
babel.core.UnknownLocaleError: unknown locale 'oc'
make: *** [Makefile:239: dev] Error 1

Comments

Since oc is currently only 21% translated, this is not a blocker for #6798; just a consequence of #6557 that will need to be addressed by freedomofpress/securedrop-engineering#30.

[cfm]

freedomofpress/securedrop-dev-docs#85 teaches us that we'll get support for oc when we update to babel>=2.10 for CLDR ≥ 41.