Netcat's behaviour after seeing EOF from stdin seems to not be very
portable or consistent, even under the same platform.
This has caused t-05-null_address to break recently under some
conditions, for example depending on the particular Debian version of
netcat-openbsd used, and the current situation is unclear.
See https://bugs.debian.org/854292 and https://bugs.debian.org/849192
for more details.
To stop depending on this brittle behaviour, this patch unfortunately
introduces a simple python3-based netcat for our tests to use.
