Fix direct instantiation of webagg_core managers.

manager_class and _toolbar2_class were correctly set for the webagg and
nbagg subclasses, but the embedding_webagg_sgskip example directly
instantiates a webagg_core manager, so it needs these attributes too.

Author: anntzer

lib/matplotlib/backends/backend_webagg_core.py

@@ -155,6 +155,7 @@ def _timer_set_interval(self):
 
 
 class FigureCanvasWebAggCore(backend_agg.FigureCanvasAgg):
+    manager_class = _api.classproperty(lambda cls: FigureManagerWebAgg)
     _timer_cls = TimerAsyncio
     # Webagg and friends having the right methods, but still
     # having bugs in practice.  Do not advertise that it works until
@@ -426,7 +427,7 @@ def set_history_buttons(self):
 
 
 class FigureManagerWebAgg(backend_bases.FigureManagerBase):
-    ToolbarCls = NavigationToolbar2WebAgg
+    _toolbar2_class = ToolbarCls = NavigationToolbar2WebAgg
 
     def __init__(self, canvas, num):
         self.web_sockets = set()