Allow testing with nbconvert 6 again (unpin)

Closes jupytergh-5086
Closes jupytergh-5092

Author: takluyver

setup.py

@@ -107,7 +107,7 @@
         'jupyter_core>=4.6.0',
         'jupyter_client>=5.3.4',
         'nbformat',
-        'nbconvert<6',
+        'nbconvert',
         'ipykernel', # bless IPython kernel for now
         'Send2Trash',
         'terminado>=0.8.1',