Release v0.6.0

[ggprior]

Automated release PR for v0.6.0, cut from a85960a.

Merging this tags v0.6.0 and starts the publish workflow, which
uploads to TestPyPI, installs the result from there to verify it, and
then publishes to PyPI.

The PyPI step runs in the pypi environment. Whether it pauses for a
review depends on that environment carrying a required reviewer, which
is repository configuration rather than anything this workflow can
assert -- an environment with no protection rules publishes
unattended.

[ggprior]

We'll do this Monday to also get #382 .

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