https://github.com/bitcoin/bitcoin/pull/13715 introduced a new check
for _transport.is_closing() in mininode's P2PConnection's. This function
is only available from Python 3.4.4, though, while Bitcoin 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
https://github.com/bitcoin/bitcoin/issues/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 https://github.com/bitcoin/bitcoin/issues/13745.
64b9f27e0e