Revert "Remove Python 2 support for PRESUBMIT.py"

This reverts commit 8454fc2458b2421e0e339714c6ff7e6fffb70dc4.

Reason for revert: post submit hooks failing
https://crbug.com/1422416

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
Bug: 1422416
Change-Id: Iaf3102e63ec3c698d0258fac5746dbd92c30edbb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4317176
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
diff --git a/git_cl.py b/git_cl.py
index e68951e..51a5708 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1435,11 +1435,21 @@
             ' was not specified. To enable ResultDB, please run the command'
             ' again with the --realm argument to specify the LUCI realm.')
 
-    return self._RunPresubmit(args,
-                              description,
-                              use_python3=True,
-                              resultdb=resultdb,
-                              realm=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)
 
   def _RunPresubmit(self,
                     args,