vimPlugins.vim-isort: fix build

isort dropped Python 2 support in 5.0.0.

pkgs/applications/editors/vim/plugins/overrides.nix

@@ -1101,7 +1101,7 @@ self: super: {
   vim-isort = super.vim-isort.overrideAttrs (old: {
     postPatch = ''
       substituteInPlace ftplugin/python_vimisort.vim \
-        --replace 'import vim' 'import vim; import sys; sys.path.append("${python2.pkgs.isort}/${python2.sitePackages}")'
+        --replace 'import vim' 'import vim; import sys; sys.path.append("${python3.pkgs.isort}/${python3.sitePackages}")'
     '';
   });