pythonPackages.graphviz: fix tests

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

···
 , mock
 , pytestCheckHook
 , pytest-mock
-, pytestcov
 }:
 
 buildPythonPackage rec {
···
     })
   ];
 
+  postPatch = ''
+    sed -i "/--cov/d" setup.cfg
+  '';
+
   # Fontconfig error: Cannot load default config file
   FONTCONFIG_FILE = makeFontsConf {
     fontDirectories = [ freefont_ttf ];
   };
 
-  checkInputs = [ mock pytestCheckHook pytest-mock pytestcov ];
+  checkInputs = [ mock pytestCheckHook pytest-mock ];
+
+  preCheck = ''
+    export HOME=$TMPDIR
+  '';
 
   meta = with lib; {
     description = "Simple Python interface for Graphviz";