Add Guild._fetch_role() private method to support proper handling of role mentionable in slash command options

[krittick]

Summary

This adds a new private method Guild._fetch_role() which allows utils.get_or_fetch to work with roles properly. Previously it did not work because there was only a fetch_roles method, not fetch_role. As part of this change, utils.get_or_fetch was also updated to use private _fetch methods if an AttributeError is raised for any object that does not have a fetch_* method. This has an added benefit of allowing us to more easily add private _fetch methods on other objects if we need to in the future.

This also fixes #906 by having the "mentionable" option type check for first a user, then a role if a user is not found. This required the above described addition of Guild._fetch_fole() to work properly.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• 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, typehinting, examples, ...)

[zeffo]

Perhaps the note should be changed to This method is an API call. For general usage, consider using :attr:`get_role` instead.

[BobDotCom]

I feel like making this a private method "_fetch_role" would be more correct (And add handling for that inside get_or_fetch). This currently doesn't reflect the api well and shouldn't really be a method we promote, as it's rather "fetch_roles_and_filter_for_one_role"

[BobDotCom]

See my last comment

[krittick]

I feel like making this a private method "_fetch_role" would be more correct (And add handling for that inside get_or_fetch). This currently doesn't reflect the api well and shouldn't really be a method we promote, as it's rather "fetch_roles_and_filter_for_one_role"

Good point. I'll update with those changes and test it out. Setting to draft for now.

stale

[krittick]

This is ready for review now.

[Icebluewolf]

I downloaded this branch and It fixed the problem for me!