[requirements] require nbconvert >=4.2

nbconvert exporters declared in setuptools entry_points were introduced only in
nbconvert 4.2, so we need at least 4.2 for thwm to work as described in our docs.

conda.recipe/meta.yaml

@@ -27,7 +27,7 @@ requirements:
     - jupyter_highlight_selected_word >=0.0.10
     - jupyter_latex_envs >=1.3.8
     - jupyter_nbextensions_configurator >=0.2.4
-    - nbconvert
+    - nbconvert >=4.2
     - notebook >=4.0
     - psutil >=2.2.1
     - pyyaml

setup.py

@@ -67,7 +67,7 @@ def main():
             'jupyter_highlight_selected_word >=0.0.10',
             'jupyter_latex_envs >=1.3.8',
             'jupyter_nbextensions_configurator >=0.2.4',
-            'nbconvert',
+            'nbconvert >=4.2',
             'notebook >=4.0',
             'psutil >=2.2.1',
             'pyyaml',