[PATCH 4 of 4] py3: fix a syntax error in concurrent.futures._base

[Matt Harbison]

# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1536874569 14400
#      Thu Sep 13 17:36:09 2018 -0400
# Node ID 21b7873d62322737c5850d07162fe8fdaf98fbf5
# Parent  9fa7b40298e9759ae76b9272a26dda00aa3b9a6e
py3: fix a syntax error in concurrent.futures._base

3.7.0 on Windows complained about this.  Not sure if there's a better way,
because check-code bans the two arg form of this.

diff --git a/mercurial/thirdparty/concurrent/futures/_base.py b/mercurial/thirdparty/concurrent/futures/_base.py
--- a/mercurial/thirdparty/concurrent/futures/_base.py
+++ b/mercurial/thirdparty/concurrent/futures/_base.py
@@ -413,7 +413,7 @@ class Future(object):
                 exception_type = self._exception.__class__
             else:
                 exception_type = type(self._exception)
-            raise exception_type, self._exception, self._traceback
+            raise (exception_type, self._exception, self._traceback)
         else:
             return self._result