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

[tacaswell]

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

I really hope there is a better way to do this.

[mdboom]

I don't know if there's a better way as long as Python3 is going to insist that:

('%s' % b'foo') == "b'foo'"

This seems like a fine solution.

[jkseppan]

I agree, looks like a good solution.

One suggestion though: I think our test suite would have had a chance of catching this (assuming it exercises the ps backend) if the io.StringIO object here was replaced with six.StringIO. In Python 2, it's exactly the same, but in Python 3 it raises an exception if you try to output a byte string.

[mdboom]

Unfortunately, we don't output the byte string directly, we do "%s" % byte_string on it, so even that wouldn't have caught it -- except running it through ghostscript would have presumably raised an error.

[jkseppan]

Oh, right. It actually becomes a Unicode string representation of the bytestring syntax. That wouldn't have helped, then.

[tacaswell]

@matplotlib/developers Can this get reviewed? This should probably go in for the (now apparently delayed) 1.4.1.