Release 1.9.0 #267

Release checklist

• Check outstanding issues on GitHub.
• Check latest documentation looks fine.
• Create a release branch.
  • Change current development version in CHANGELOG.rst to stable version.
• Check if the address sanitizer does not find any problems using tox -e asan
• Merge the release branch into main.
• Created an annotated tag with the stable version number. Include changes
  from CHANGELOG.rst.
• Push tag to remote. This triggers the wheel/sdist build on GitHub CI.
• merge main branch back into develop.
• Build the new tag on readthedocs. Only build the last patch version of
  each minor version. So 1.1.1 and 1.2.0 but not 1.1.0, 1.1.1 and 1.2.0.
• Create a new release on GitHub.
• Update the package on conda-forge.