python.pkgs.pygal: fix build (#53833)

`pytestrunner` is a required build input.

Also we need to explicitly set the locale to pass the tests on darwin.

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

···
 , flask
 , pyquery
 , pytest
+, pytestrunner
 , cairosvg
 , tinycss
 , cssselect
···
     sha256 = "9204f05380b02a8a32f9bf99d310b51aa2a932cba5b369f7a4dc3705f0a4ce83";
   };
 
-  buildInputs = [ flask pyquery pytest ];
+  buildInputs = [
+    flask
+    pyquery
+
+    # Should be a check input, but upstream lists it under "setup_requires".
+    # https://github.com/Kozea/pygal/issues/430
+    pytestrunner
+  ];
+
+  checkInputs = [
+    pytest
+  ];
+
+  preCheck = ''
+    # necessary on darwin to pass the testsuite
+    export LANG=en_US.UTF-8
+  '';
+
   propagatedBuildInputs = [ cairosvg tinycss cssselect ]
     ++ stdenv.lib.optionals (!isPyPy) [ lxml ];