Fix missing scikit-learn-extra dependency in GitHub Actions tests

Add scikit-learn-extra to testing optional dependencies to ensure it's
available when running tests via tox in CI environment.

Author: macromeer

pyproject.toml

@@ -51,6 +51,7 @@ testing = [
     "napari",
     "pyqt5",
     "psygnal>=0.8.0",
+    "scikit-learn-extra>=0.3.0",
 ]
 clustering = [
     "scikit-learn-extra>=0.3.0",