Only mock tornado is micropip is available

Also, use a more specific error exception

Author: ianhi

ipympl/backend_nbagg.py

@@ -10,8 +10,10 @@
 # solution than a non-working solution here.
 try:
     import micropip  # noqa
-except ImportError:
+
     sys.modules['tornado'] = types.ModuleType('tornadofake')
+except ModuleNotFoundError:
+    pass
 
 import io
 import json