[jupyter] check existence of TWebCanvas methods

Looks like dummy TWebCanvas class is created
by pyroot also when it not compiled.
So extend check and verify that static method is
really exists.
Do the same for TBufferJSON

Author: linev

bindings/jupyroot/python/JupyROOT/helpers/utils.py

@@ -89,16 +89,20 @@
 
 
 def TBufferJSONAvailable():
-    if hasattr(ROOT, "TBufferJSON"):
-        return True
-    print(TBufferJSONErrorMessage, file=sys.stderr)
-    return False
+    if not hasattr(ROOT, "TBufferJSON"):
+        return False
+    if not hasattr(ROOT.TBufferJSON, "ConvertToJSON"):
+        print(TBufferJSONErrorMessage, file=sys.stderr)
+        return False
+    return True
 
 
 def TWebCanvasAvailable():
-    if hasattr(ROOT, "TWebCanvas"):
-        return True
-    return False
+    if not hasattr(ROOT, "TWebCanvas"):
+        return False
+    if not hasattr(ROOT.TWebCanvas, "CreateCanvasJSON"):
+        return False
+    return True
 
 
 def RCanvasAvailable():