#13715 introduced a new check for `_transport.is_closing()` in mininode's `P2PConnection`'s.  This function is [only available from Python 3.4.4](https://docs.python.org/3.4/library/asyncio-protocol.html#asyncio.BaseTransport.is_closing), though, while Bitcoin Core is supposed to support all Python 3.4 versions.

In this change, we make the check conditional on `is_closing` being available.  If it is not, then we revert to the behaviour before the check was introduced; this means that #13579 is not fixed for old systems, but at least the tests work as they used to do before.

This includes a small refactoring from a one-line lambda to an inline function, because this makes the code easier to read with more and more conditions being added.

Fixes #13745.