Release gsd 3.3.1

[joaander]

Release checklist:

• [x] Run bumpversion.
• [x] Review the change log.
• [x] Check for new or duplicate contributors since the last release: comm -13 (git log $(git describe --tags --abbrev=0) --format="%aN <%aE>" | sort | uniq | psub) (git log --format="%aN <%aE>" | sort | uniq | psub). Add entries to .mailmap to remove duplicates.
• [x] Check readthedocs build, especially change log formatting.
  • Build status
  • Output
• [x] Tag and push.
• [ ] Update conda-forge recipe.