Don't force unicode strings for UUID coercion

Change Ic6b6308fb1960ec40407e6efde30137b64543e72 attempts to
fix difference between Python 2 and Python 3 values by switching
from using str() to formatting into a unicode string (u"%s").

This is equivalent to changing str() to unicode(), but that is
not correct for expected default string types for Python 2.
This requires either using six.text_type(), or just formatting
into a string, without forcing unicode ("%s"), to be correct on
either runtime.

Change-Id: I178f14cdc670d7a696778891e587ef75de208fc2
Closes-bug: #1763179

oslo_versionedobjects/fields.py

@@ -352,7 +352,7 @@ class UUID(StringPattern):
             # like 'error' for this warning.
             warnings.filterwarnings(action="once", append=True)
             try:
-                uuid.UUID(u"%s" % value)
+                uuid.UUID("%s" % value)
             except Exception:
                 # This is to ensure no breaking behaviour for current
                 # users
@@ -367,7 +367,7 @@ class UUID(StringPattern):
                               repr(value).encode('utf8'),
                               FutureWarning)
 
-            return u"%s" % value
+            return "%s" % value
 
 
 class MACAddress(StringPattern):