Fix the PDF build of the documentation

[bouweandela]

Description

Fix the PDF build of the documentation, similar to ESMValGroup/ESMValCore#3068

• Link to documentation builds: https://app.readthedocs.org/projects/esmvaltool/builds/?version__slug=fix-pdf-build
• Link to PDF: https://docs.esmvaltool.org/_/downloads/en/fix-pdf-build/pdf/

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[bettina-gier]

Looks good