`cgi` is deprecated as of Python 3.11

There are two uses of the `cgi` module in treq:

1. One use of `cgi.parse_header()` in `treq.content` &mdash; [suggested replacement is `email.message.Message`](https://peps.python.org/pep-0594/#cgi)
2. One use of `cgi.parse_multipart()` in `treq.test.test_multipart` &mdash; same suggested replacement
