[py3] Remove py2 support

https://crrev.com/c/3782569 adds support to the recipe engine for the
`py3_only` recipes.cfg field, which removes the requirement that recipe
code be parseable by the Python 2 interpreter.

We no longer need the `PYTHON_VERSION_COMPATIBILITY` annotations, so
I'll remove them in a follow-up.

Change-Id: I5ec0fb49b4bfceacf445fed69c1ba36d854c6326
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/32142
Commit-Queue: Rob Mohr <mohrr@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
diff --git a/infra/config/recipes.cfg b/infra/config/recipes.cfg
index cd4f3a7..bf8b14e 100644
--- a/infra/config/recipes.cfg
+++ b/infra/config/recipes.cfg
@@ -41,5 +41,6 @@
   },
   "no_docs": true,
   "project_id": "flutter",
+  "py3_only": true,
   "repo_name": "flutter"
 }