build(deps): bump vobject from 0.9.6.1 to 0.9.7

Bumps [vobject](https://github.com/py-vobject/vobject) from 0.9.6.1 to 0.9.7.
- [Release notes](https://github.com/py-vobject/vobject/releases)
- [Commits](py-vobject/vobject@0.9.6.1...v0.9.7)

---
updated-dependencies:
- dependency-name: vobject
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/optional.txt

@@ -24,4 +24,4 @@ python-Levenshtein>=0.12    # Levenshtein
 # Format support
 ruamel.yaml==0.18.6 # YAML
 # Format support
-vobject==0.9.6.1     # iCal
+vobject==0.9.7     # iCal