Automatically remove "awaiting core review" label for draft and stale PRs

[eendebakpt]

The short story

There are many open PRs with the label "awaiting core review" (currently: 475). To keep the list of items to be addressed by core developers manageable I suggest to apply the following rules

1. For a PR marked as draft the label "awaiting core review" should not be added. If a PR changes status to draft, the label should be removed. If a PR changes status from draft to non-draft, the normal rules for applying an "awaiting core review" label should apply.
2. If a PR has merge conflicts and has gone stale, the label "awaiting core review" should be removed.
3. If a PR is stale for some longer time period (e.g. 2 months), the label should be removed.

When removing the label, perhaps a short message can be added by the bot explaining the action (and explaining how to get back the label, e.g. rebase)

Long version

The first rule above was suggested on discourse:

discuss.python.org: A request to review a PR for fixing an issue on the turtle module