Prepare Orca 1.9 release

[waddell]

Summary

Prepares Orca 1.9 for release.

• changes the package version from 1.9.dev0 to 1.9;
• names the pending changelog section v1.9; and
• updates the documentation version and copyright year.

Orca 1.8 was published to PyPI in October 2022. Version 1.9 is therefore the
next release, although the 1.8 Git tag and GitHub release record still need to
be restored separately.

Verification

• 74 tests pass on Python 3.12 with current dependencies.
• Ruff passes.
• The source distribution and wheel build successfully.
• twine check passes for both artifacts.

This pull request prepares the source tree only. It does not create a tag,
publish a GitHub release, or upload a package to PyPI.