ea5340c9d2 tests: fixes mininode's P2PConnection sending messages on closing transport (marcoagner)
Pull request description:
Fixes #13579. I think one possible solution is to check for [`_transport.is_closing()`](https://docs.python.org/3.4/library/asyncio-protocol.html#asyncio.BaseTransport.is_closing) in the lambda before sending a message (compatible with Python 3.4 too). Let me know if I missed any side effects this introduces.
Tree-SHA512: cab46f81dccfec7b4460fda478a617845564520694449a9e85bf8a5f1e75f35f52cafd7c64966712c3d6c29956344d5a9dbad8851424f061eb3748bc621b900b
Backport of Core PR13715
https://github.com/bitcoin/bitcoin/pull/13715/files
Completes T678
Depends on D4012