don't fork on Windows

pymc3/tests/test_parallel_sampling.py

@@ -13,6 +13,7 @@
 #   limitations under the License.
 import multiprocessing
 import os
+import platform
 
 import aesara
 import aesara.tensor as at
@@ -97,6 +98,11 @@ def test_abort(mp_start_method):
 
     step = pm.CompoundStep([step1, step2])
 
+    # on Windows we cannot fork
+    if platform.system() == "Windows" and mp_start_method == "fork":
+        raise ValueError(
+            "this can be ignored but raising error to satisfy xfail; replace with 'return' once test starts passing"
+        )
     if mp_start_method == "spawn":
         step_method_pickled = cloudpickle.dumps(step, protocol=-1)
     else:
@@ -135,6 +141,9 @@ def test_explicit_sample(mp_start_method):
 
     step = pm.CompoundStep([step1, step2])
 
+    # on Windows we cannot fork
+    if platform.system() == "Windows" and mp_start_method == "fork":
+        return
     if mp_start_method == "spawn":
         step_method_pickled = cloudpickle.dumps(step, protocol=-1)
     else: