python3Packages.pygments: enable tests

this requires a similar trick as used by hypothesis to get around pytest
circular dependency

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

@@ -2,6 +2,8 @@
 , buildPythonPackage
 , fetchPypi
 , docutils
+, pytestCheckHook
+, doCheck ? true
 }:
 
 buildPythonPackage rec {
@@ -15,8 +17,8 @@ buildPythonPackage rec {
 
   propagatedBuildInputs = [ docutils ];
 
-  # Circular dependency with sphinx
-  doCheck = false;
+  inherit doCheck;
+  checkInputs = [ pytestCheckHook ];
 
   meta = {
     homepage = "https://pygments.org/";