Add 'mention' property in PartialWebhookChannel

[Gooraeng]

Summary

I was trying to make a function to follow channel(s) and found PartialWebhookChannel can be set when source_channel is not None. So this means source_channel_id also would not be None. So I would like to suggest to add mention property for this.

I hope you'll review it positively. Thank you.

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, ...)

[Rapptz]

The changeset seems fine but you need to run black to pass CI.

Thanks.

[Gooraeng]

I ran black --line-length 125 --skip-string-normalization discord/webhook/async_.py, but I still have same problem. Should I close and reopen pr again?

[Rapptz]

We use an older version of black, the 22.6 version so try using that instead.

[Gooraeng]

Thanks. It's ready for being merged.