Allow "hg arc-ls-markers" to run under Python 2 or Python 3

Summary:
Ref T13546. See PHI1805. Currently, the "arc-ls-markers" extension doesn't run under Python 3:

  - some stuff needs "b'...'" to mark it as a byte string;
  - "dict.iteritems()" is gone in Python 3, and "mercurial.pycompat" isn't always available;
  - in Python 3, "json" refuses to print byte strings; and
  - the compiler caching behavior in Python 3 has changed.

Try to get these things working in the same way under Python 2 and Python 3.

Test Plan:
Ran this command (with `python` as Python 2, locally):

```
$ python /usr/local/bin/hg --config 'extensions.arc-hg=/Users/epriestley/dev/core/lib/arcanist/support/hg/arc-hg.py' arc-ls-markers --
```

...and this command:

```
$ python3 /usr/local/bin/hg --config 'extensions.arc-hg=/Users/epriestley/dev/core/lib/arcanist/support/hg/arc-hg.py' arc-ls-markers --
```

..and saw the same output in both cases (previously, `python3 ...` fataled in various ways).

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21392

.gitignore

@@ -36,3 +36,4 @@
 
 # Python extension compiled files.
 /support/hg/arc-hg.pyc
+/support/hg/__pycache__/

support/hg/arc-hg.py

@@ -1,4 +1,14 @@
 from __future__ import absolute_import
+import sys
+
+is_python_3 = sys.version_info[0] >= 3
+
+if is_python_3:
+  def arc_items(dict):
+    return dict.items()
+else:
+  def arc_items(dict):
+    return dict.iteritems()
 
 import os
 import json
@@ -19,8 +29,8 @@ cmdtable = {}
 command = registrar.command(cmdtable)
 
 @command(
-  "arc-ls-markers",
+  b'arc-ls-markers',
-  [('', 'output', '',
+  [(b'', b'output', b'',
     _('file to output refs to'), _('FILE')),
   ] + cmdutil.remoteopts,
   _('[--output FILENAME] [SOURCE]'))
@@ -42,6 +52,16 @@ def lsmarkers(ui, repo, source=None, **opts):
   else:
     markers = remotemarkers(ui, repo, source, opts)
 
+  for m in markers:
+    if m['name'] != None:
+      m['name'] = m['name'].decode('utf-8')
+
+    if m['node'] != None:
+      m['node'] = m['node'].decode('utf-8')
+
+    if m['description'] != None:
+      m['description'] = m['description'].decode('utf-8')
+
   json_opts = {
     'indent': 2,
     'sort_keys': True,
@@ -54,14 +74,15 @@ def lsmarkers(ui, repo, source=None, **opts):
     with open(output_file, 'w+') as f:
       json.dump(markers, f, **json_opts)
   else:
-    print json.dumps(markers, output_file, **json_opts)
+    json_data = json.dumps(markers, **json_opts)
+    print(json_data)
 
   return 0
 
 def localmarkers(ui, repo):
   markers = []
 
-  active_node = repo['.'].node()
+  active_node = repo[b'.'].node()
   all_heads = set(repo.heads())
   current_name = repo.dirstate.branch()
   saw_current = False
@@ -117,7 +138,7 @@ def localmarkers(ui, repo):
   bookmarks = repo._bookmarks
   active_bookmark = repo._activebookmark
 
-  for bookmark_name, bookmark_node in bookmarks.iteritems():
+  for bookmark_name, bookmark_node in arc_items(bookmarks):
     is_active = (active_bookmark == bookmark_name)
     description = repo[bookmark_node].description()