automation: run hg with python3

Python 2.7 support will go away soon. Let's use Python 3 as part of
the automation.

Differential Revision: https://phab.mercurial-scm.org/D12260

contrib/automation/hgautomation/linux.py

@@ -532,7 +532,7 b' def synchronize_hg('
         hg_bin = source_path / 'hg'
 
         res = subprocess.run(
-            ['python2.7', str(hg_bin), 'log', '-r', revision, '-T', '{node}'],
+            ['python3', str(hg_bin), 'log', '-r', revision, '-T', '{node}'],
             cwd=str(source_path),
             env=env,
             check=True,
@@ -542,7 +542,7 b' def synchronize_hg('
         full_revision = res.stdout.decode('ascii')
 
         args = [
-            'python2.7',
+            'python3',
             str(hg_bin),
             '--config',
             'ui.ssh=ssh -F %s' % ssh_config,

contrib/automation/hgautomation/windows.py

@@ -261,7 +261,7 b' def synchronize_hg(hg_repo: pathlib.Path'
         hg_bin = hg_repo / 'hg'
 
         res = subprocess.run(
-            ['python2.7', str(hg_bin), 'log', '-r', revision, '-T', '{node}'],
+            ['python3', str(hg_bin), 'log', '-r', revision, '-T', '{node}'],
             cwd=str(hg_repo),
             env=env,
             check=True,
@@ -271,7 +271,7 b' def synchronize_hg(hg_repo: pathlib.Path'
         full_revision = res.stdout.decode('ascii')
 
         args = [
-            'python2.7',
+            'python3',
             hg_bin,
             '--config',
             'ui.ssh=ssh -F %s' % ssh_config,