Fix Novus on Python 3.11

[shinrax2]

Summary

This PR fixes Novus running on Python 3.11.0 by making use of default_factory for fields inside of discord.ext.commands.flags.Flag.
I tested this PR on Python 3.8.10, 3.9.13, 3.10.7 & 3.11.0 with the reproduction code from #61 and my bot (which is quite basic) and had no errors.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[4Kaylum]

I tested this PR on Python 3.8.10, 3.9.13, 3.10.7 & 3.11.0 with the reproduction code from #61 and my bot (which is quite basic) and had no errors.

Did you just see if the bot launched, or did you test the affected functionality?

The change you've made will set name, attribute, etc to MISSING() (ie _MissingSentinel()()). This is not a syntax error as it was before, but still not valid when called.

Your change should be field(default_factory=lambda: MISSING). Could you make that change?

[GlitchMasta47]

These changes should be valid