Update classifier in setup.cfg

[phackstock]

Update the classifier Topic :: Climate Science to the closest allowed value by pypi, Topic :: Scientific/Engineering :: Atmospheric Science (list of all allowed classifiers: https://pypi.org/classifiers/). This is necessary to be able to upload to pypi. Not sure this needs any of the usual PR steps.

• [ ] Tests added
• [ ] Documentation added
• [ ] Example added (in the documentation, to an existing notebook, or in a new notebook)
• [ ] Description in CHANGELOG.rst added (single line such as: (`#XX <https://github.com/iiasa/climate-assessment/pull/XX>`_) Added feature which does something)

[znicholls]

Could also be worth adding a test from pypi workflow for when you've deployed (e.g. here but I'm sure you've done this elsewhere too https://github.com/openscm/openscm-units/blob/master/.github/workflows/test-pypi-install.yml)

[phackstock]

@znicholls, thank you for the suggestions, that was going to be my next PR to complete the deployment setup.