glib-mkenums: fix encoding error when writing files

Instead of using NamedTemporaryFile, which doesn't take an encoding in Python 2
use mkstemp() to create a file and open it with io.open(), with a proper
encoding set.

https://bugzilla.gnome.org/show_bug.cgi?id=785113

gobject/glib-mkenums.in

@@ -366,7 +366,9 @@ if output is not None:
     out_suffix = '_' + os.path.splitext(out_fn)[1]
     if out_dir == '':
         out_dir = '.'
-    tmpfile = tempfile.NamedTemporaryFile(dir=out_dir, delete=False)
+    fd, filename = tempfile.mkstemp(dir=out_dir)
+    os.close(fd)
+    tmpfile = io.open(filename, "w", encoding="utf-8")
     output_stream = tmpfile
 else:
     tmpfile = None