update sample_label attribute with list import

[HaleySchuhl]

Describe your changes
append labels to sample_label attribute during .import_list method.

Type of update
Is this a:

• Bug fix
• New feature or feature enhancement
• Update to documentation
• Work in progress

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

Additional context

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 c2ed906..f5b003d. 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%	100%

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