Add napari backend pyside2 to environment.yml

[k034b363]

Describe your changes
When creating an environment using conda-forge, the default installed backend for napari doesn't work anymore. This PR adds the alternate backend pyside2 to the environment file.

Type of update
Is this a:

• Bug fix

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 changelog.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 447f2ab..2c98af6. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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