pylint: roll six to 1.15.0.

Matching chromium. (I'm sure this is asking for trouble.)

Bug: chromium:1007101
Change-Id: Iba3200bd093697f41551c99f58a9c5261c958436
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2726729
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>

pylint-1.5

@@ -48,7 +48,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.10.0"
+#   version: "version:1.15.0"
 # >
 #
 # # Pylint extensions.

pylint-1.6

@@ -48,7 +48,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.10.0"
+#   version: "version:1.15.0"
 # >
 #
 # # Pylint extensions.

pylint-1.7

@@ -48,7 +48,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.10.0"
+#   version: "version:1.15.0"
 # >
 #
 # # Pylint extensions.

pylint-1.8

@@ -48,7 +48,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.10.0"
+#   version: "version:1.15.0"
 # >
 #
 # # Pylint extensions.

pylint-1.9

@@ -48,7 +48,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.10.0"
+#   version: "version:1.15.0"
 # >
 #
 # # Pylint extensions.

pylint-2.6

@@ -24,7 +24,7 @@
 # >
 # wheel: <
 #   name: "infra/python/wheels/six-py2_py3"
-#   version: "version:1.12.0"
+#   version: "version:1.15.0"
 # >
 # wheel: <
 #   name: "infra/python/wheels/toml-py3"