Migrate firebase recipe module to python3.

Change-Id: Idd215d9e1a2e9813cc8b9ebfedf11429676847d1
Bug: crbug/1256513
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/19681
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>
diff --git a/recipe_modules/firebase/api.py b/recipe_modules/firebase/api.py
index f7af450..067a8c1 100644
--- a/recipe_modules/firebase/api.py
+++ b/recipe_modules/firebase/api.py
@@ -5,6 +5,9 @@
 from recipe_engine import recipe_api
 
 
+PYTHON_VERSION_COMPATIBILITY = 'PY2+3'
+
+
 class FirebaseApi(recipe_api.RecipeApi):
   """Provides utilities to upload docs to Firebase.