Update pyproject.toml (until next plantcv version release)

[HaleySchuhl]

Describe your changes
Resolve errors in github actions by pinning the version of altair to pre-v6. This will be resolved by the change to pyproject.toml that took place in the main plantcv package, but until there is a pip version release, plantcv-geospatial will continue to install plantcv with the latest version of altair unless limited in its own pyproject.toml.

Type of update
Is this a:

• Bug fix

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

Additional context
Add any other context about the problem here.

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv-geospatial/mkdocs.yml
• Changes to function input/output signatures added to changelog.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 79c39da..f44cf67. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%

---

💡 If you’re a repository administrator, you can configure the quality gates from the settings.