python3: make qt image test work on python3

QByteArrays are natively bytes and ended up looking like this: u"b'xpm'"
Instead use the builtin method to get the data back, and decode it to
unicode.

src/calibre/test_build.py

@@ -175,7 +175,7 @@ class BuildTest(unittest.TestCase):
         # it should just work because the hard-coded paths of the Qt
         # installation should work. If they do not, then it is a distro
         # problem.
-        fmts = set(map(unicode_type, QImageReader.supportedImageFormats()))
+        fmts = set(map(lambda x: x.data().decode('utf-8'), QImageReader.supportedImageFormats()))
         testf = {'jpg', 'png', 'svg', 'ico', 'gif'}
         self.assertEqual(testf.intersection(fmts), testf, "Qt doesn't seem to be able to load some of its image plugins. Available plugins: %s" % fmts)
         data = P('images/blank.png', allow_user_override=False, data=True)