Cap opencv version

[joshqsumner]

Describe your changes
New installations of PlantCV will get opencv 4.13 which breaks automatic color card detection. We have a fix in v5.0 but until that is ready for release it seems simplest to cap the opencv version at 4.12.0.

Type of update
This is a stopgap solution to a potential problem until v5 is released.

Associated issues
None, but see tripsicum seed head tutorial for error from opencv 4.13.0

Additional context
Had a fix in 1871 pointed at v5 but may need to revisit.

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/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

DeepSource Code Review

DeepSource reviewed changes in the commit range 43ce1be..22c386d on this pull request. Below is the summary for the review, and you can see the individual issues we found as review comments.

For detailed review results, please see the PR on DeepSource ↗

PR Report Card

Security	× 0 issues		Overall PR Quality
Reliability	× 0 issues
Complexity	× 0 issues
Hygiene	× 0 issues
Coverage	× 0 issues

Code Review Summary

Analyzer	Status	Summary	Details
Python		No new issues detected.	Review ↗
Test coverage		No new issues detected.	Review ↗

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Code Coverage Summary

Language	Line Coverage	Branch Coverage
Aggregate	100%	100%
Python	100%	100%

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