```
Traceback (most recent call last):
  File "env/bin/pybabel", line 8, in <module>
    sys.exit(main())
  File "env/lib/python3.8/site-packages/babel/messages/frontend.py", line 1031, in main
    return CommandLineInterface().run(sys.argv)
  File "env/lib/python3.8/site-packages/babel/messages/frontend.py", line 955, in run
    return cmdinst.run()
  File "env/lib/python3.8/site-packages/babel/messages/frontend.py", line 521, in run
    for filename, lineno, message, comments, context in extracted:
  File "env/lib/python3.8/site-packages/babel/messages/extract.py", line 222, in check_and_call_extract_file
    for message_tuple in extract_from_file(
  File "env/lib/python3.8/site-packages/babel/messages/extract.py", line 258, in extract_from_file
    return list(extract(method, fileobj, keywords, comment_tags,
  File "env/lib/python3.8/site-packages/babel/messages/extract.py", line 340, in extract
    for lineno, funcname, messages, comments in results:
  File "liberapay/i18n/extract.py", line 17, in extract_custom
    for match in extractor(*args, **kw):
  File "env/lib/python3.8/site-packages/babel/messages/extract.py", line 496, in extract_python
    value = eval(code, {'__builtins__': {}}, {})
  File "<string>", line 2, in <module>
NameError: name 'self' is not defined
```