Documentation build on CircleCI is failing

[bouweandela]

The documentation build is failing with the new version of sphinx because it emits a long list of warnings, see below for the first few warnings. I couldn't find in the sphinx autosummary documentation how to solve this.

Related issue: sphinx-doc/sphinx#12589.

WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels' with 'CheckLevels'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CMORCheck' with 'CMORCheck'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CMORCheckError' with 'CMORCheckError'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.cmor_check_metadata' with 'cmor_check_metadata'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.cmor_check_data' with 'cmor_check_data'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.cmor_check' with 'cmor_check'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.DEBUG' with 'DEBUG'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.DEBUG' with 'CheckLevels.DEBUG'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.STRICT' with 'STRICT'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.STRICT' with 'CheckLevels.STRICT'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.DEFAULT' with 'DEFAULT'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.DEFAULT' with 'CheckLevels.DEFAULT'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.RELAXED' with 'RELAXED'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.RELAXED' with 'CheckLevels.RELAXED'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.IGNORE' with 'IGNORE'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CheckLevels.IGNORE' with 'CheckLevels.IGNORE'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CMORCheck.check_metadata' with 'check_metadata'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CMORCheck.check_metadata' with 'CMORCheck.check_metadata'.
WARNING: Summarised items should not include the current module. Replace 'esmvalcore.cmor.check.CMORCheck.check_data' with 'check_data'.
...