Use execnet main_thread_only execmodel (#1027)

Use the execnet main_thread_only execmodel so that code which expects
to run in the main thread will just work.  This execmodel has been
merged to the execnet master branch via pytest-dev/execnet#243, so this
patch should not be merged until there is a released version of execnet
supporting the main_thread_only execmodel.

Closes #620

src/xdist/looponfail.py

@@ -82,7 +82,7 @@ class RemoteControl:
             print("RemoteControl:", msg)
 
     def initgateway(self) -> execnet.Gateway:
-        return execnet.makegateway("popen")
+        return execnet.makegateway("execmodel=main_thread_only//popen")
 
     def setup(self) -> None:
         if hasattr(self, "gateway"):