codegen: Use six.PY3 rather than six.PY2

Older versions of python-six (at least 1.3.0) defined six.PY3 but not
six.PY2. six.PY2 is only used twice in straightforward tests so it's
easy to use six.PY3 instead.

python_modules/codegen.py

@@ -121,10 +121,10 @@ class CodeWriter:
 
     def write(self, s):
         # Ensure its a unicode string
-        if six.PY2:
-            s = unicode(s)
-        else:
+        if six.PY3:
             s = str(s)
+        else:
+            s = unicode(s)
 
         if len(s) == 0:
             return

spice_codegen.py

@@ -265,10 +265,10 @@ if options.keep_identical_file:
         pass
 
 f = open(dest_file, 'wb')
-if six.PY2:
-    f.write(content)
-else:
+if six.PY3:
     f.write(bytes(content, 'UTF-8'))
+else:
+    f.write(content)
 f.close()
 
 six.print_("Wrote %s" % dest_file)