Python 3 forbids unbuffered I/O in text mode. Change the reading of
stdin in git-remote-testpy so that we read the lines as bytes and then
decode them a line at a time.
This allows us to keep the I/O unbuffered in order to avoid
reintroducing the bug fixed by commit 7fb8e16 (git-remote-testgit: fix
race when spawning fast-import).
Signed-off-by: John Keeping <john@keeping.me.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>