Add an `open_actor_cluster()` usage example

examples/quick_cluster.py

@@ -0,0 +1,50 @@
+from functools import partial
+
+import trio
+import tractor
+
+
+async def sleepy_jane():
+    uid = tractor.current_actor().uid
+    print(f'Yo i am actor {uid}')
+    await trio.sleep_forever()
+
+
+async def main():
+    '''
+    Spawn a flat actor cluster, with one process per
+    detected core.
+
+    '''
+    portal_map: dict[str, tractor.Portal]
+    results: dict[str, str]
+
+    # look at this hip new syntax!
+    async with (
+
+        tractor.open_actor_cluster(
+            modules=[__name__]
+        ) as portal_map,
+
+        trio.open_nursery() as n,
+    ):
+
+        for (name, portal) in portal_map.items():
+            n.start_soon(
+                partial(
+                    portal.run,
+                    sleepy_jane,
+                )
+            )
+
+        await trio.sleep(0.5)
+
+        # kill the cluster with a cancel
+        raise KeyboardInterrupt
+
+
+if __name__ == '__main__':
+    try:
+        trio.run(main)
+    except KeyboardInterrupt:
+        pass