Readable exclude pattern for pre-commit

[stefsmeets]

Small tweak to make the excluded files span multiple lines, as in ESMValGroup/ESMValTool#1805 . It makes the regex a bit more robust, looks nicer, and will make it a bit easier to add/remove new directories to the exclude list if necessary.

Tasks

• This pull request has a descriptive title that can be used in a changelog
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
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• Please use yamllint to check that your YAML files do not contain mistakes

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