Add PYTHON_VERSION_COMPATIBILITY to gerrit recipe module

Bug: 1227140
Change-Id: I3cea6609eb5bb6b7f392e1078eb023ca19c6fb3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3123939
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

recipes/recipe_modules/gerrit/__init__.py

@@ -1,3 +1,5 @@
+PYTHON_VERSION_COMPATIBILITY = 'PY2+3'
+
 DEPS = [
     'recipe_engine/context',
     'recipe_engine/file',

recipes/recipe_modules/gerrit/examples/full.py

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+PYTHON_VERSION_COMPATIBILITY = 'PY2+3'
+
 DEPS = [
     'gerrit',
     'recipe_engine/step',