Add imagehash package as an ESMValTool dependency

[alistairsellar]

Description

For recipe_test_workflow, and also fixing the errors that occurred when merging #3435, add imagehash package as a dependency.

3436 talked about "development dependency", but think that "test" section may be more appropriate, since used by esmvaltool/utils/testing/regression/compare.py which is for test purposes. Test appears to be sufficient for inclusion in docker build also.

• Closes #3436

• Link to documentation: https://esmvaltool--3557.org.readthedocs.build/en/3557

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[alistairsellar]

@valeriupredoi does this cover what is needed for adding imagehash as a dev/test dependency?

I've done everything in https://docs.esmvaltool.org/en/latest/community/code_documentation.html#dependencies but should I check anything else?