Update d type in fill_holes.py check

[eseigel]

Changing data type check for catching grayscale and rgb while still allowing a pure black/white mask

Describe your changes
A clear and concise description of what changes are made by this pull request.
What was the previous functionality (if relevant) and what can we do now with
these changes.

Type of update
Bug fix

Associated issues
#1635

Additional context
N/A

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
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 4995a6e..f7ca5d6. 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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