chore: release v0.3.1

[github-actions]

🤖 New release

• hugr: 0.3.0 -> 0.3.1 (✓ API compatible changes)

Changelog

0.3.1 (2024-04-23)

Features

• new methods for leaf ops (#940)
• CustomOp::extension utility function (#951)
• Encoder metadata in serialized hugr (#955)

Testing

• Add a test of instantiating an extension set (#939)

---

This PR was generated with release-plz.

[codecov]

Codecov Report

Attention: Patch coverage is 88.74172% with 17 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (patch/hugr-v0.3.1@68c8ded). Click here to learn what that means.

❗ Current head 1345b15 differs from pull request most recent head 179992c. Consider uploading reports for the commit 179992c to get more accurate results

Files	Patch %	Lines
.../std_extensions/arithmetic/float_ops/const_fold.rs	23.07%	9 Missing and 1 partial ⚠️
hugr/src/extension/prelude.rs	91.66%	3 Missing and 1 partial ⚠️
hugr/src/hugr/rewrite/replace.rs	96.42%	0 Missing and 2 partials ⚠️
hugr/src/ops/constant.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##             patch/hugr-v0.3.1     #928   +/-   ##
====================================================
  Coverage                     ?   85.85%           
====================================================
  Files                        ?       78           
  Lines                        ?    14384           
  Branches                     ?    14384           
====================================================
  Hits                         ?    12350           
  Misses                       ?     1398           
  Partials                     ?      636           

Flag	Coverage Δ
rust	85.85% <88.74%> (?)

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[aborgna-q]

release-plz doesn't really support non-default branches.

For this PR I created a patch branch locally with the required changes,
and then ran release-plz release-pr --git-token ** locally.

This created this PR, targeting main and including all of main PRs into the release-plz branch.
I had to manually change the target branch, and drop the imported commits.
I also ran semver-checks locally to be sure that there are no breaking changes.

The changelog looks correct, but the workflow doesn't quite work yet.