[FIX] web: pie chart structure not modified by favorites

A pie chart widget aims to instantiate a graph view in mode
'pie', with a measure and a single groupby specified via some
attributes. It turns out that if a pie chart is instantiated
in a context in which the keys 'graph_mode', 'graph_measure',
and 'graph_groupbys' are present, that keys prevail over the
pie chart specification. This can produce a line chart for
instance. This fix corrects that situation.

A test has been added in a corresponding commit in enterprise
(historically, the pie chart widgets are tested in dashboard
views).

closes odoo/odoo#34234

Signed-off-by: VincentSchippefilt <VincentSchippefilt@users.noreply.github.com>

addons/web/static/src/js/widgets/pie_chart.js

@@ -34,8 +34,13 @@ var PieChart = Widget.extend({
             title: node.attrs.title || modifiers.title || modifiers.measure,
         });
 
+        var pieChartContext = JSON.parse(JSON.stringify(record.context));
+        delete pieChartContext.graph_mode;
+        delete pieChartContext.graph_measure;
+        delete pieChartContext.graph_groupbys;
+
         this.subViewParams = {
-            context: record.context,
+            context: pieChartContext,
             domain: domain,
             groupBy: [],
             modelName: record.model,