Update closing.py

[eseigel]

Changed len(np.unique(gray_img)) to >2

Describe your changes
Changed len(np.unique(gray_img)) to >2, allowing all black or white images to be treated as binary.

Type of update
Is this a:
New feature or feature enhancement

Associated issues
#1635

Additional context
Add any other context about the problem here.

For the reviewer
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• 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
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• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 432fcd2..4eaefbd. 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%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

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