bump min matplotlib version

This was already too low - previously was 3.3.1. With update of
doubleclick we now need to require 3.4

Author: ianhi

setup.py

@@ -64,7 +64,7 @@
         'pillow',
         'traitlets<6',
         'ipywidgets>=7.6.0,<8',
-        'matplotlib>=2.0.0,<4',
+        'matplotlib>=3.4.0,<4',
     ],
     extras_require={
         "docs": [