[build] update gen-postmortem-metadata for Python 3

This change makes the code compatible with both Python 2 and Python 3.

Change-Id: I99d68af9c3163607c3a2fdbafac339a98b7471e4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1751331
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63207}

tools/gen-postmortem-metadata.py

@@ -653,9 +653,7 @@ def emit_config():
 
         out.write('/* class type information */\n');
         consts = [];
-        keys = typeclasses.keys();
-        keys.sort();
-        for typename in keys:
+        for typename in sorted(typeclasses):
                 klass = typeclasses[typename];
                 consts.append({
                     'name': 'type_%s__%s' % (klass, typename),
@@ -666,9 +664,7 @@ def emit_config():
 
         out.write('/* class hierarchy information */\n');
         consts = [];
-        keys = klasses.keys();
-        keys.sort();
-        for klassname in keys:
+        for klassname in sorted(klasses):
                 pklass = klasses[klassname]['parent'];
                 bklass = get_base_class(klassname);
                 if (bklass != 'Object'):