Roll luci/client-py to tip of tree.

Recent changes to luci/client-py contain functionality improvement/fixes to
swarming.py. This is needed by v8 recipes to dispatch swarming tasks.

Bug: chromium:934482
Change-Id: Icdbf213918f10d5bc67d6ce078179e95888281e5
Reviewed-on: https://chromium-review.googlesource.com/c/1480920Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59784}

DEPS

@@ -62,7 +62,7 @@ deps = {
   'v8/third_party/markupsafe':
     Var('chromium_url') + '/chromium/src/third_party/markupsafe.git' + '@' + '8f45f5cfa0009d2a70589bcda0349b8cb2b72783',
   'v8/tools/swarming_client':
-    Var('chromium_url') + '/infra/luci/client-py.git' + '@' + '0e3e1c4dc4e79f25a5b58fcbc135dc93183c0c54',
+    Var('chromium_url') + '/infra/luci/client-py.git' + '@' + 'd50a88f50782ba29076061b94c7b9d08a6c7e424',
   'v8/test/benchmarks/data':
     Var('chromium_url') + '/v8/deps/third_party/benchmarks.git' + '@' + '05d7188267b4560491ff9155c5ee13e207ecd65f',
   'v8/test/mozilla/data':