Fix RTD version discovery

[bjlittle]

🐛 Bug Report

How To Reproduce

The recent adoption of setuptools-scm from PR #4841 is causing the incorrect iris version to be rendered on the iris readthedocs (RTD).

In addition, when building the documentation on RTD the version is tagged as +dirty, which although is an side-effect of how RTD builds the publishes the documentation, is misleading i.e., we should parse out the +dirty extension to the version.

Expected behaviour

The current version should be rendered as something like v3.2.2.dev158.

Screenshots

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