fix: remove use of deprecated cgi module

[adamchainz]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #892 🦕

[adamchainz]

Comparison of the two approaches:

>>> import cgi
<stdin>:1: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13

>>> from email.message import Message

>>> content_type = 'text/plain; charset=utf-8; other=something'

>>> cgi.parse_header(content_type)[1]
{'charset': 'utf-8', 'other': 'something'}

>>> message = Message(); message['content-type'] = content_type; dict(message.get_params()[1:])
{'charset': 'utf-8', 'other': 'something'}

[adamchainz]

Moved to HeaderParser, checked for equivalence on shell again:

>>> from email.parser import HeaderParser

>>> content_type = 'text/plain; charset=utf-8; other=something'

>>> msg = HeaderParser().parsestr("Content-Type: " + content_type)

>>> dict(msg.get_params()[1:])
{'charset': 'utf-8', 'other': 'something'}

[andrewsg]

Thank you for doing this! I'll have the tests run against this branch and we'll go from there.

[andrewsg]

Thank you!