Change pytest version to 3.6.2 to align with upstream WPT runner.

Bug: 881513
Change-Id: I7545077ccf34528b9053561749f65bd1553e8ca2
Reviewed-on: https://chromium-review.googlesource.com/c/1474600
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Kyle Ju <kyleju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633824}

.vpython

@@ -232,7 +232,7 @@ wheel: <
 
 wheel <
   name: "infra/python/wheels/pytest-py2_py3"
-  version: "version:3.5.0"
+  version: "version:3.6.2"
 >
 
 wheel <
@@ -257,7 +257,7 @@ wheel <
 
 wheel <
   name: "infra/python/wheels/pluggy-py2_py3"
-  version: "version:0.6.0"
+  version: "version:0.7.1"
 >
 
 wheel <
@@ -278,3 +278,13 @@ wheel: <
   name: "infra/python/wheels/colorama-py2_py3"
   version: "version:0.4.1"
 >
+
+wheel: <
+  name: "infra/python/wheels/atomicwrites-py2_py3"
+  version: "version:1.3.0"
+>
+
+wheel: <
+  name: "infra/python/wheels/pathlib2-py2_py3"
+  version: "version:2.3.3"
+>