Actually documentation doesn't say that it supports only bytes, but:

"The following methods support packing strings, bytes, and opaque data:"

Also under python2 you can easily do this:

In [1]: import xdrlib

In [2]: p = xdrlib.Packer()

In [3]: p.pack_string('some str')

In [4]: p.pack_string(u'some str')

So to conclude I believe either docs for python3 should be changed to say that only bytes are allowed or it should be changed to work as in python2. It's clear that in python2 some unicode string are accepted and I think we should allow such strings to be accepted in python3 too.