python3Packages.fastjsonschema: fix Darwin build

Test was failing on hydra build, with this error:
```
ModuleNotFoundError: No module named 'temp.schema_3'

tests/test_compile_to_code.py:95: ModuleNotFoundError
```

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

@@ -1,4 +1,5 @@
 { lib
+, stdenv
 , buildPythonPackage
 , fetchFromGitHub
 , pytestCheckHook
@@ -31,6 +32,8 @@ buildPythonPackage rec {
     # these tests require network access
     "remote ref"
     "definitions"
+  ] ++ lib.optionals stdenv.isDarwin [
+    "test_compile_to_code_custom_format"  # cannot import temporary module created during test
   ];
 
   pythonImportsCheck = [