4523: Migrate to pathlib.Path

[SgtVarmint]

Closes #4523 in migrating uses of os.path to pathlib.Path where possible.

[CLAassistant]

All committers have signed the CLA.

[scitools-ci]

Templating

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• docs/src/conf.py

Footnotes

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   @scitools-templating: please no update notification on: iris ↩