(subplotlib): Do not generate tests for subplotlib if not in-tree

Daniel Silverstonerequested to merge
fix-release into main

Description

This fixes an issue where subplotlib couldn't build out of tree, and prepares a 0.4.1 release for it

Checklist

  • You have read, and followed the guidance in, the CONTRIBUTING guide
  • The MR content has run through a pipeline, or at least has pipelines enabled.
  • There are no licence concerns with this contribution (The README clearly states Subplot is MIT-0)

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