python3.pkgs.ruamel-base: add pythonNamespaces to remove nspkg.pth file

pkgs/development/python-modules/ruamel-base/default.nix

···
 
   pythonImportsCheck = [ "ruamel.base" ];
 
+  pythonNamespaces = [ "ruamel" ];
+
   meta = with lib; {
     description = "Common routines for ruamel packages";
     homepage = "https://sourceforge.net/projects/ruamel-base/";