Revert "Use py3 in gitiles recipe module"

This reverts commit 0faae1c8.

Reason for revert: broke dawn and some chromeos builds

Original change's description:
> Use py3 in gitiles recipe module
>
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: chrome_release
> Recipe-Nontrivial-Roll: chromiumos
> Bug: 1289280
> Change-Id: I182f0e7ad1da58c1c707edd15675f6d30c5faed8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3438327
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
> Commit-Queue: Gavin Mak <gavinmak@google.com>

Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: chrome_release
Recipe-Nontrivial-Roll: chromiumos
Bug: 1289280
Change-Id: I09f2860bac9fbac4c2edb0b77baea3802b077958
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3444000
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/gitiles/api.py

@@ -57,8 +57,8 @@ class Gitiles(recipe_api.RecipeApi):
       args.extend([
           '--accept-statuses',
           ','.join([str(s) for s in accept_statuses])])
-    cmd = ['python3', '-u', self.resource('gerrit_client.py')] + args
-    return self.m.step(step_name, cmd, **kwargs)
+    return self.m.python(
+        step_name, self.resource('gerrit_client.py'), args, **kwargs)
 
   def refs(self, url, step_name='refs', attempts=None):
     """Returns a list of refs in the remote repository."""

recipes/recipe_modules/gitiles/examples/full.expected/basic.json

 [
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -27,7 +27,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -136,7 +136,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -246,7 +246,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -354,7 +354,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -462,7 +462,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -515,7 +515,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -531,7 +531,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -549,7 +549,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",
@@ -568,7 +568,7 @@
   },
   {
     "cmd": [
-      "python3",
+      "python",
       "-u",
       "RECIPE_MODULE[depot_tools::gitiles]/resources/gerrit_client.py",
       "--json-file",

recipes/recipe_modules/gitiles/resources/gerrit_client.py

-#!/usr/bin/env python3
+#!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.