Require nbformat explicitly

requirements.txt

@@ -24,6 +24,7 @@ karabo_data == 0.7.0
 lxml == 4.5.0
 metadata_client == 3.0.5
 nbconvert == 5.6.1
+nbformat == 5.0.7
 prettytable == 0.7.2
 pypandoc == 1.4
 python-dateutil == 2.8.1