Let the bot give people triage access

[asmeurer]

There is a new "triage" permission in GitHub, which gives people the ability to add labels to issues without giving them push access. See https://help.github.com/en/articles/repository-permission-levels-for-an-organization.

The only issue is that you have to add people to it automatically. We could let the bot give people triage access.

One option would be to have a special bot command like "@sympy-bot give @user triage access". A problem with this is that it isn't discoverable or even easy to remember, so it probably wouldn't get used.

Another option would be to give everyone who opens a pull request access. This could be a bit intrusive, but maybe not. You do have to accept an invitation to actually get the access, and I think you can remove yourself at any time (but I should confirm this).

If this is too much we could have the bot ask people if they want it, or maybe have a link in the bot message to a page where they can enter their username, and the bot will give it. Probably if we do it that way we should authenticate with GitHub so that people can only give themselves access. Otherwise it could be used to spam people.

[asmeurer]

Here is the relevant API https://developer.github.com/v3/teams/members/#add-or-update-team-membership

If I understand the permissions correctly, we need to make the bot a "team maintainer" for the "Triage" team. That way it will have access to add new people. Otherwise I think it would have to have admin privileges for SymPy, which would be unacceptable.