Revert "Migrate //PRESUBMIT*.py to Python3."

This reverts commit 8b59f97def70a451cb491d0bdd1819ba4840deda.

Reason for revert:
Breaks uploading CLs which change localized strings. See
https://bugs.chromium.org/p/chromium/issues/detail?id=1209392.

Original change's description:
> Migrate //PRESUBMIT*.py to Python3.
>
> This CL migrates the root-level PRESUBMIT checks, their associated
> tests, and and one other file that got pulled along for the ride)
> to run under Python3 instead of Python2.
>
> Bug: 816629
> Change-Id: I75f3edb34ca72458432ba54f3afa022e027f8eb9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2877897
> Commit-Queue: Dirk Pranke <[email protected]>
> Reviewed-by: Andrew Grieve <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#883002}

Bug: 816629
Fixed: 1209392
Change-Id: Icf0c084f1db3d20ca76600eb67cc3f6e81b3c9f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2899404
Auto-Submit: Kyle Horimoto <[email protected]>
Commit-Queue: Rubber Stamper <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Owners-Override: Yutaka Hirano <[email protected]>
Cr-Commit-Position: refs/heads/master@{#883359}
diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index eb78f61..f8143ae 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -126,7 +126,7 @@
       if file_.LocalPath() == filename:
         return '\n'.join(file_.NewContents())
     # Otherwise, file is not in our mock API.
-    raise IOError("No such file or directory: '%s'" % filename)
+    raise IOError, "No such file or directory: '%s'" % filename
 
 
 class MockOutputApi(object):