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/nbconvert/default.nix

@@ -47,7 +47,7 @@ buildPythonPackage rec {
     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
   ];