Return str rather than bytes for the description strings on python3.

Assumes utf-8 encoding from magic return values, which I hope is
always the case.

magic.py

@@ -72,7 +72,7 @@ def from_buffer(self, buf):
         """
         with self.lock:
             try:
-                return magic_buffer(self.cookie, buf)
+                return maybe_decode(magic_buffer(self.cookie, buf))
             except MagicException as e:
                 return self._handle509Bug(e)
 
@@ -82,7 +82,7 @@ def from_file(self, filename):
             pass
         with self.lock:
             try:
-                return magic_file(self.cookie, filename)
+                return maybe_decode(magic_file(self.cookie, filename))
             except MagicException as e:
                 return self._handle509Bug(e)
 
@@ -189,6 +189,14 @@ def errorcheck_negative_one(result, func, args):
         return result
 
 
+# return str on python3.  Don't want to unconditionally
+# decode because that results in unicode on python2
+def maybe_decode(s):
+    if str == bytes:
+        return s
+    else:
+        return s.decode('utf-8')
+
 def coerce_filename(filename):
     if filename is None:
         return None

test/test.py

@@ -22,13 +22,11 @@ def assert_values(self, m, expected_values):
                 expected_value = (expected_value,)
 
             for i in expected_value:
-                expected_value_bytes = i.encode('utf-8')
-
                 with open(filename, 'rb') as f:
                     buf_value = m.from_buffer(f.read())
 
                 file_value = m.from_file(filename)
-                if buf_value == expected_value_bytes and file_value == expected_value_bytes:
+                if buf_value == i and file_value == i:
                     break
             else:
                 self.assertTrue(False, "no match for " + repr(expected_value))
@@ -86,11 +84,10 @@ def test_keep_going(self):
         filename = os.path.join(self.TESTDATA_DIR, 'keep-going.jpg')
 
         m = magic.Magic(mime=True)
-        self.assertEqual(m.from_file(filename), 
-                         'image/jpeg'.encode('utf-8'))
+        self.assertEqual(m.from_file(filename), 'image/jpeg')
 
         m = magic.Magic(mime=True, keep_going=True)
-        self.assertEqual(m.from_file(filename), 'image/jpeg'.encode('utf-8'))
+        self.assertEqual(m.from_file(filename), 'image/jpeg')
 
 
     def test_rethrow(self):