Make pycapnp GIL friendly

Now calling `wait` from one thread will not block all threads

test/test_threads.py

@@ -0,0 +1,66 @@
+import capnp
+import pytest
+import test_capability_capnp
+import socket
+import threading
+import platform
+
+
+def test_making_event_loop():
+    capnp.remove_event_loop(True)
+    capnp.create_event_loop()
+
+    capnp.remove_event_loop()
+    capnp.create_event_loop()
+
+
+def test_making_threaded_event_loop():
+    capnp.remove_event_loop(True)
+    capnp.create_event_loop(True)
+
+    capnp.remove_event_loop()
+    capnp.create_event_loop(True)
+
+
+class Server(test_capability_capnp.TestInterface.Server):
+
+    def __init__(self, val=1):
+        self.val = val
+
+    def foo(self, i, j, **kwargs):
+        return str(i * 5 + self.val)
+
+
+class SimpleRestorer(test_capability_capnp.TestSturdyRefObjectId.Restorer):
+
+    def restore(self, ref_id):
+        assert ref_id.tag == 'testInterface'
+        return Server(100)
+
+
+@pytest.mark.skipif(platform.python_implementation() == 'PyPy', reason="pycapnp's GIL handling isn't working properly at the moment for PyPy")
+def test_using_threads():
+    capnp.remove_event_loop(True)
+    capnp.create_event_loop(True)
+
+    read, write = socket.socketpair(socket.AF_UNIX)
+
+    def run_server():
+        restorer = SimpleRestorer()
+        server = capnp.TwoPartyServer(write, restorer)
+        capnp.wait_forever()
+
+    server_thread = threading.Thread(target=run_server)
+    server_thread.daemon = True
+    server_thread.start()
+
+    client = capnp.TwoPartyClient(read)
+
+    ref = test_capability_capnp.TestSturdyRefObjectId.new_message(tag='testInterface')
+    cap = client.restore(ref)
+    cap = cap.cast_as(test_capability_capnp.TestInterface)
+
+    remote = cap.foo(i=5)
+    response = remote.wait()
+
+    assert response.x == '125'