git-remote-testpy: fix patch hashing on Python 3

When this change was originally made (0846b0c - git-remote-testpy: hash bytes
explicitly , I didn't realised that the "hex" encoding we chose is a "bytes to
bytes" encoding so it just fails with an error on Python 3 in the same way as
the original code.

Since we want to convert a Unicode string to bytes, UTF-8 really is the best
option here.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>