Fix nbformat to 5.2.0 to cell None type (#770)

Upgrading nbformat, as nbformat==5.1.2 and 5.1.3 causes "AttributeError: 'NoneType' object has no attribute 'cells'" due to upgrading nbformat to v4 `nb = nbformat.v4.upgrade(nb)`

requirements.txt

@@ -1,6 +1,6 @@
 click
 pyyaml
-nbformat >= 5.1.2
+nbformat >= 5.2.0
 nbclient >= 0.2.0
 tqdm >= 4.32.2
 requests