fix: Disable STAN_THREADS test

PyStan 2.20 will not support using STAN_THREADS as there appears no way
to support it using gcc. Trying to use a model compiled with
STAN_THREADS with gcc yields an immediate segfault.

Author: riddell-stan

pystan/tests/test_extra_compile_args.py

@@ -40,6 +40,7 @@ def test_extra_compile_args_failure(self):
             pystan.StanModel(model_code=model_code, model_name="normal1",
                              extra_compile_args=extra_compile_args)
 
+    @unittest.skip("temporarily disabled due to STAN_THREADS issue in Stan 2.20")
     def test_threading_support(self):
         # Dont test with Windows with MinGW-w64 (GCC)
         if sys.platform.startswith("win"):