Version 1.9

[smmaurer]

Sets the version to 1.9 for the final release, following the 1.9rc1 dry run (#69).

After merging

• Tag v1.9 on the merge commit and publish a GitHub release (not a pre-release) with the changelog text as the notes
• Approve the pypi deployment when the Publish workflow pauses for it
• Check that pip install orca resolves to 1.9 in a fresh environment
• Review and merge the conda-forge feedstock PR once the bot opens it (the recipe needs updating; see the notes there)
• Rebuild and publish the documentation to gh-pages