Test suite were ported to python3, however, twistd binary is still required
for ftp test cases. The twistd binary is part of python2-twisted, which is
going to be retired. Let's use python3 version instead. It seems that Fedora
has twistd-3 binary, whereas, Debian has twistd3, so try to look for both of
them. The test cases seem working nicely with it as per my testing.

Fixes: #428