Fixed testing against Pytest 5.1

As of Pytest 5.1.0
> pytest.raises and pytest.warns no longer support strings as the
> second argument

https://docs.pytest.org/en/latest/changelog.html#removals

Fixes: #106
Signed-off-by: Stanislav Levin <[email protected]>

Author: stanislavlevin

testing/test_gateway.py

@@ -36,10 +36,10 @@ def test_deprecation(recwarn, monkeypatch):
     execnet.PopenGateway().exit()
     assert recwarn.pop(DeprecationWarning)
     monkeypatch.setattr(socket, "socket", fails)
-    py.test.raises(Exception, 'execnet.SocketGateway("localhost", 8811)')
+    py.test.raises(Exception, execnet.SocketGateway, "localhost", 8811)
     assert recwarn.pop(DeprecationWarning)
     monkeypatch.setattr(subprocess, "Popen", fails)
-    py.test.raises(Exception, 'execnet.SshGateway("not-existing")')
+    py.test.raises(Exception, execnet.SshGateway, "not-existing")
     assert recwarn.pop(DeprecationWarning)