(stable) 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