include nbformat as a plotly[express] dependency

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Author: afeld

CHANGELOG.md

@@ -4,6 +4,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Updated
+
+- Include `nbformat` as a dependency for `plotly[express]`. [[#5380](https://github.com/plotly/plotly.py/pull/5380)]
+
 ## [6.3.1] - 2025-10-02
 
 ### Updated

pyproject.toml

@@ -39,7 +39,10 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-express = ["numpy"]
+express = [
+    "numpy",
+    "nbformat>=4.2.0"
+]
 kaleido = ["kaleido>=1.0.0"]
 dev_core = [
     "pytest",