python3.pkgs.nbclient: always disable tests

Overriding a Python package in propagatedBuildInputs is a bad idea as
you may end up with multiple versions in your closure. This happened,
and when installing to $out the package for which the tests were run,
the site packages folder was empty.

Possible improvement is to add the tests as a separate derivation.

pkgs/development/python-modules/nbclient/default.nix

···
 { lib, buildPythonPackage, fetchPypi, pythonOlder,
   async_generator, traitlets, nbformat, nest-asyncio, jupyter-client,
   pytest, xmltodict, nbconvert, ipywidgets
-, doCheck ? true
 }:
 
 buildPythonPackage rec {
···
     sha256 = "sha256-QMUsm148MfrsruafICs/U+ONfBxWPeD63enX7aD9r+g=";
   };
 
-  inherit doCheck;
+  doCheck = false; # Avoid infinite recursion
   checkInputs = [ pytest xmltodict nbconvert ipywidgets ];
   propagatedBuildInputs = [ async_generator traitlets nbformat nest-asyncio jupyter-client ];
 

pkgs/development/python-modules/nbconvert/default.nix

···
     entrypoints bleach mistune jinja2 pygments traitlets testpath
     jupyter_core nbformat ipykernel pandocfilters tornado jupyter-client
     defusedxml beautifulsoup4
-    (nbclient.override { doCheck = false; }) # avoid infinite recursion
+    nbclient
     jupyterlab-pygments
   ];