Reland "Remove Python 2 support for PRESUBMIT.py"

This is a reland of commit 8454fc2458b2421e0e339714c6ff7e6fffb70dc4

Original change's description:
> Remove Python 2 support for PRESUBMIT.py
>
> The presubmit system still supports invoking PRESUBMIT.py files using
> Python 2. This has recently been turned off on the bots so this change
> removes support more completely.
>
> There are still some python3 parameters being passed around - it seemed
> better to do the simplest possible removal now, with a follow-up change
> to remove more support code after this has sat for a while.
>
> Tests run from PRESUBMIT.py files could still be run using Python 2, but
> those should also have been addressed already. Removing support for that
> will be done in a subsequent change.
>
> Bug: 1207012
> Change-Id: Id244d547a04438f83734dba269c3cc180c148b37
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4315183
> Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

Bug: 1207012
Change-Id: If542cac21d8ec8704b28d03fd8407c5c2899ca2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4317177
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 51a5708..dae123d 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1435,21 +1435,11 @@
             ' was not specified. To enable ResultDB, please run the command'
             ' again with the --realm argument to specify the LUCI realm.')
 
-    py3_results = self._RunPresubmit(args,
-                                     description,
-                                     use_python3=True,
-                                     resultdb=resultdb,
-                                     realm=realm)
-    if py3_results.get('skipped_presubmits', 1) == 0:
-      print('No more presubmits to run - skipping Python 2 presubmits.')
-      return py3_results
-
-    py2_results = self._RunPresubmit(args,
-                                     description,
-                                     use_python3=False,
-                                     resultdb=resultdb,
-                                     realm=realm)
-    return self._MergePresubmitResults(py2_results, py3_results)
+    return self._RunPresubmit(args,
+                              description,
+                              use_python3=True,
+                              resultdb=resultdb,
+                              realm=realm)
 
   def _RunPresubmit(self,
                     args,
@@ -1489,19 +1479,6 @@
         json_results = gclient_utils.FileRead(json_output)
         return json.loads(json_results)
 
-  def _MergePresubmitResults(self, py2_results, py3_results):
-    return {
-        'more_cc': sorted(set(py2_results.get('more_cc', []) +
-                              py3_results.get('more_cc', []))),
-        'errors': (
-            py2_results.get('errors', []) + py3_results.get('errors', [])),
-        'notifications': (
-            py2_results.get('notifications', []) +
-            py3_results.get('notifications', [])),
-        'warnings': (
-            py2_results.get('warnings', []) + py3_results.get('warnings', []))
-    }
-
   def RunPostUploadHook(self, verbose, upstream, description, py3_only):
     args = self._GetCommonPresubmitArgs(verbose, upstream)
     args.append('--post_upload')
@@ -1509,13 +1486,8 @@
     with gclient_utils.temporary_file() as description_file:
       gclient_utils.FileWrite(description_file, description)
       args.extend(['--description_file', description_file])
-      if not py3_only:
-        p_py2 = subprocess2.Popen(['vpython', PRESUBMIT_SUPPORT] + args)
-      p_py3 = subprocess2.Popen(['vpython3', PRESUBMIT_SUPPORT] + args +
-                                ['--use-python3'])
-      if not py3_only:
-        p_py2.wait()
-      p_py3.wait()
+      subprocess2.Popen(['vpython3', PRESUBMIT_SUPPORT] + args +
+                        ['--use-python3']).wait()
 
   def _GetDescriptionForUpload(self, options, git_diff_args, files):
     # type: (optparse.Values, Sequence[str], Sequence[str]