Fix pep8-naming errors and fix zmnam diagnostic

[bouweandela]

Fix pep8-naming errors introduced with the latest version of pep8-naming (can be seen in the failing nightly development test on CircleCI) and fix zmnam diagnostic:

• Make zmnam work with cartopy 0.18
• Fix provenance of zmnam

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Tasks

• Give this pull request a descriptive title that can be used as a one line summary 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.
• Preferably Codacy code quality checks pass, however a few remaining hard to solve Codacy issues are still acceptable. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

[valeriupredoi]

was just about to ask about those flakey8 tests 🍺