Avoid setting signals when not in main thread (and warn)

#153

Author: altendky

pytest_twisted.py

@@ -200,7 +200,16 @@ def init_twisted_greenlet():
         return
 
     if not _instances.reactor.running:
-        if signal.getsignal(signal.SIGINT) == signal.default_int_handler:
+        if not isinstance(threading.current_thread(), threading._MainThread):
+            warnings.warn(
+                (
+                    'Failed to block Twisted signal configuration due to not running in the main thread.'
+                    '  See https://github.com/pytest-dev/pytest-twisted/issues/153.'
+
+                ),
+                RuntimeWarning,
+            )
+        elif signal.getsignal(signal.SIGINT) == signal.default_int_handler:
             signal.signal(
                 signal.SIGINT,
                 functools.partial(signal.default_int_handler),