Revert "Drop py2 support in recipe modules"

This reverts commit 5819c303f0.

Reason for revert: breaking dart builds

Original change's description:
> Drop py2 support in recipe modules
>
> python3 is the only supported version of python in depot_tools.
>
> Bug: 1475402
> Change-Id: I479de09a0c34b438aced35e4ced58a5972108132
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4808518
> Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
> Commit-Queue: Gavin Mak <gavinmak@google.com>

Bug: 1475402
Change-Id: I0fef7581835bb25ea7e4f22303eef11e72df9536
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4811596
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

recipes/recipe_modules/gerrit/__init__.py

@@ -1,4 +1,4 @@
-PYTHON_VERSION_COMPATIBILITY = 'PY3'
+PYTHON_VERSION_COMPATIBILITY = 'PY2+3'
 
 DEPS = [
     'recipe_engine/context',

recipes/recipe_modules/gerrit/examples/full.py

@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-PYTHON_VERSION_COMPATIBILITY = 'PY3'
+PYTHON_VERSION_COMPATIBILITY = 'PY2+3'
 
 DEPS = [
     'gerrit',