contrib: simplify the genosxversion.py command to find the hg libraries

I forget what problem I ran into while trying to teach the makefile to use a
non-system python.  (It might have ben missing hg-evolve and/or keyring, but
`check_output()` was raising an error.)  This still isn't great because it will
return non zero for something like the username not being set, even though we
aren't asking for it.  But I suppose it's still useful to simplify.

Differential Revision: https://phab.mercurial-scm.org/D6753

contrib/genosxversion.py

@@ -2,14 +2,13 b''
 from __future__ import absolute_import, print_function
 
 import argparse
-import json
 import os
 import subprocess
 import sys
 
 # Always load hg libraries from the hg we can find on $PATH.
-hglib = json.loads(subprocess.check_output(
-    ['hg', 'debuginstall', '-Tjson']))[0]['hgmodules']
+hglib = subprocess.check_output(
+    ['hg', 'debuginstall', '-T', '{hgmodules}'])
 sys.path.insert(0, os.path.dirname(hglib))
 
 from mercurial import util