pytest_30: Add override from pytest_27

pkgs/top-level/python-packages.nix

···
     };
   };
 
+  pytest_30 = self.pytest_27.override rec {
+    name = "pytest-3.0.3";
+
+    propagatedBuildInputs = with self; [ hypothesis py ];
+    src = pkgs.fetchurl {
+      url = "mirror://pypi/p/pytest/${name}.tar.gz";
+      sha256 = "1rxydacrdb8s312l3bn0ybrqsjp13abzyim1x21s80386l5504zj";
+    };
+  };
+
   pytestcache = buildPythonPackage rec {
     name = "pytest-cache-1.0";
     src = pkgs.fetchurl {