Revert "Update Presubmit to python3"

This reverts commit 1b7204d841.

Reason for revert: py2 presubmits no longer running, and some
aren't migrated to run py3 yet.

Original change's description:
> Update Presubmit to python3
>
> Recipe-Nontrivial-Roll: build
> Bug:1370463
> Change-Id: I0bf182badc16f0d75c7330f52906f4cf9ff66e08
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3975294
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Riley Wong <rgw@google.com>

Change-Id: I82c3f0f0d502ee896d4e79a6bb330aee71b4d309
Recipe-Nontrivial-Roll: build
Bug: 1370463
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4021307
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/presubmit/api.py

@@ -30,7 +30,7 @@ class PresubmitApi(recipe_api.RecipeApi):
 
     name = kwargs.pop('name', 'presubmit')
     with self.m.depot_tools.on_path():
-      cmd = ['vpython3', self.presubmit_support_path]
+      cmd = ['vpython', self.presubmit_support_path]
       cmd.extend(args)
       cmd.extend(['--json_output', self.m.json.output()])
       if self.m.resultdb.enabled:

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

@@ -1,7 +1,7 @@
 [
   {
     "cmd": [
-      "vpython3",
+      "vpython",
       "RECIPE_REPO[depot_tools]/presubmit_support.py",
       "--json_output",
       "/path/to/tmp/json"