Support running synmark on macOS. (#8578)

By using the "poll" reactor since macOS doesn't support epoll.

changelog.d/8578.misc

@@ -0,0 +1 @@
+Support macOS on the `synmark` benchmark runner.

synmark/__init__.py

@@ -15,7 +15,10 @@
 
 import sys
 
-from twisted.internet import epollreactor
+try:
+    from twisted.internet.epollreactor import EPollReactor as Reactor
+except ImportError:
+    from twisted.internet.pollreactor import PollReactor as Reactor
 from twisted.internet.main import installReactor
 
 from synapse.config.homeserver import HomeServerConfig
@@ -63,7 +66,7 @@ def make_reactor():
     Instantiate and install a Twisted reactor suitable for testing (i.e. not the
     default global one).
     """
-    reactor = epollreactor.EPollReactor()
+    reactor = Reactor()
 
     if "twisted.internet.reactor" in sys.modules:
         del sys.modules["twisted.internet.reactor"]