Update pylint vpython specs to use a specific minor version.

This is always recommended, since wheel availability varies between
minor versions. We may not support major-version-only specifiers
in the future.

Change-Id: Ib41426fa39212fe3e49d87c1d1c8b5bbc49aef7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3708295
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Brian Ryner <bryner@google.com>

pylint-2.6

@@ -9,7 +9,7 @@
 # [VPYTHON:BEGIN]
 # # Third party dependencies.  These are only listed because pylint itself needs
 # # them.  Feel free to add/remove anything here.
-# python_version: "3"
+# python_version: "3.8"
 #
 # wheel: <
 #   name: "infra/python/wheels/futures-py2_py3"

pylint-2.7

@@ -9,7 +9,7 @@
 # [VPYTHON:BEGIN]
 # # Third party dependencies.  These are only listed because pylint itself needs
 # # them.  Feel free to add/remove anything here.
-# python_version: "3"
+# python_version: "3.8"
 #
 # wheel: <
 #   name: "infra/python/wheels/futures-py2_py3"