web_widget_plotly_chart: pin plotly version

As indicated in the addon README.

setup/web_widget_plotly_chart/setup.py

@@ -2,5 +2,11 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['setuptools-odoo'],
-    odoo_addon=True,
+    odoo_addon={
+        "external_dependencies_override": {
+            "python": {
+                "plotly": "plotly==4.1.0",
+            }
+        }
+    },
 )