Add vpython dependencies needed to run presubmit tests on LUCI

E.g. tools_webrtc/libs/generate_licenses_test.py needs 'mock', which needs the other libs added here.

This is taken from 641bce4223/.vpython (29)

TBR: kwiberg@webrtc.org
No-Try: True
Bug: chromium:749664
Change-Id: I180fc1190d2664c5a82f63857b740334145e7daa
Reviewed-on: https://webrtc-review.googlesource.com/77520
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23294}

.vpython

@@ -43,3 +43,20 @@ wheel: <
     platform: "win_amd64"
   >
 >
+
+wheel: <
+  name: "infra/python/wheels/six-py2_py3"
+  version: "version:1.10.0"
+>
+wheel: <
+  name: "infra/python/wheels/pbr-py2_py3"
+  version: "version:3.0.0"
+>
+wheel: <
+  name: "infra/python/wheels/funcsigs-py2_py3"
+  version: "version:1.0.2"
+>
+wheel: <
+  name: "infra/python/wheels/mock-py2_py3"
+  version: "version:2.0.0"
+>