pythonPackages.fonttools: fix tests

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

@@ -3,6 +3,7 @@
 , numpy
 , pytest
 , pytestrunner
+, glibcLocales
 }:
 
 buildPythonPackage rec {
@@ -25,6 +26,10 @@ buildPythonPackage rec {
     glibcLocales
   ];
 
+  preCheck = ''
+    export LC_ALL="en_US.UTF-8"
+  '';
+
   meta = {
     homepage = https://github.com/fonttools/fonttools;
     description = "A library to manipulate font files from Python";