Update protobuf-py2_py3 wheel.

This is required after updating protobuf to 3.20.0 in
https://chromium-review.googlesource.com/c/chromium/src/+/3733827.

Bug: b/237760527
Change-Id: I9c4df89b7116dfa934a2c313aaad9c9b88c36787
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/267405
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#37399}

.vpython3

@@ -73,8 +73,8 @@ wheel: <
   version: "version:2.0.0"
 >
 wheel: <
-  name: "infra/python/wheels/protobuf-py2_py3"
-  version: "version:3.13.0"
+  name: "infra/python/wheels/protobuf-py3"
+  version: "version:3.20.0"
 >
 wheel: <
   name: "infra/python/wheels/requests-py2_py3"

infra/specs/trybot_analyze_config.json

@@ -1,6 +1,8 @@
 {
   "base": {
       "exclusions": [
+          ".vpython",
+          ".vpython3",
           "DEPS",
           "infra/specs/.*"
       ]