BUG : fix str vs bytes issue in py3 in ps backend

de-code the encoded ascii string so that it is a string, not
bytes, when it gets passed to the format function.  Having the
b'' from the bytes string appear in the eps file breaks the file.

closes #3494
Merge pull request #3496 from tacaswell/eps_fontname_bytes

BUG : fix str vs bytes issue in py3 in ps backend