[python3] Add missing python3 dependency

This is required for a V8 DEPS roll:
https://chromium-review.googlesource.com/c/v8/v8/+/3691119.

Change-Id: I65371eb7fd23b9dd13802e77bd3c14afdd592641
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3688407Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80978}

.vpython3

@@ -69,3 +69,8 @@ wheel: <
   name: "infra/python/wheels/numpy/${vpython_platform}"
   version: "version:1.2x.supported.1"
 >
+
+wheel: <
+  name: "infra/python/wheels/protobuf-py3"
+  version: "version:3.19.3"
+>