⬆️ UPGRADE: nbconvert 6 support (#326)

Author: bollwyvl

setup.cfg

@@ -45,7 +45,7 @@ install_requires =
     jupyter-cache~=0.4.1
     jupyter_sphinx~=0.3.2
     myst-parser~=0.14.0
-    nbconvert~=5.6
+    nbconvert>=5.6,<7
     nbformat~=5.0
     pyyaml
     sphinx>=2,<4