Change to github handling: clarify label use

[zklaus]

Using our draft_release_notes.py script, the following Labels are special in that they are used to sort the pull requests into categories.
That means that exactly one of them should be assigned to every PR. To make this clearer, I suggest to make them all the same color, and to modify the PR template checklist to split up the point "This pull request has a descriptive title and labels" into separate points "This pull request has a descriptive title" and "This pull request has exactly one (blue) label".

The list of special labels is

• bug
• deprecated feature
• documentation
• fix for dataset
• cmor
• preprocessor
• api
• testing
• installation
• enhancement

[valeriupredoi]

I'd go even further and suggest using an Action that checks the label eg https://github.com/marketplace/actions/verify-pull-request-labels

[zklaus]

I like your idea! It is more about enforcing the policy, whereas this ticket is more about changing/clarifying the policy, I would say, but perhaps we can do both.

[remi-kazeroni]

I also like these ideas! We should perhaps also update the documentation which currently says: "At least one of the following labels should be used: bug, deprecated feature, fix for dataset, preprocessor, cmor, api, testing, documentation or enhancement." Having the one and only one label checked automatically could be helpful as well.

[zklaus]

In the last release, the wish of including one and the same PR in more than one category surfaced. @ESMValGroup/esmvaltool-coreteam, how do you feel about this?

[axel-lauer]

I am fine with including the same PR in more than one category.