Issue 31641: concurrent.futures.as_completed() no longer accepts arbitrary iterables

bpo-27144 introduced a regression for `as_completed()`: it used to accept arbitrary iterables but now requires sequences, otherwise raising an exception like:


Traceback (most recent call last):
  File "sandcastle/worker/sandcastle_worker.py", line 1266, in get_work_item
  File "jupiter/jupiter_client.py", line 166, in acquireWork
  File "jupiter/jupiter_client.py", line 241, in _acquire_multiple_async
  File "jupiter/jupiter_client.py", line 336, in _wait_for_responses
  File "/usr/local/fbcode/gcc-5-glibc-2.23/lib/python3.6/concurrent/futures/_base.py", line 217, in as_completed
    total_futures = len(fs)
TypeError: object of type 'map' has no len()


Since `as_completed()` is a very popular function, we need to revert the behavior here.

Sounds like we need to get this fixed for 3.6.3 final which is scheduled for Monday (3 days).  Or revert the previous fix.

New changeset 574562c5ddb2f0429aab9af762442e6f9a3f26ab by Łukasz Langa in branch 'master':
bpo-31641: Allow arbitrary iterables in `concurrent.futures.as_completed()` (#3830)
https://github.com/python/cpython/commit/574562c5ddb2f0429aab9af762442e6f9a3f26ab

New changeset 9ef28b6ad3d5aff767e3d852499def8b5ae5ff5d by Łukasz Langa (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31641: Allow arbitrary iterables in `concurrent.futures.as_completed()` (GH-3830) (#3831)
https://github.com/python/cpython/commit/9ef28b6ad3d5aff767e3d852499def8b5ae5ff5d

Fix is already landed in 3.6 and master.

New changeset 2e3fd03796528a9f88f79d38e1d890af153c8e61 by Ned Deily (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31641: Allow arbitrary iterables in `concurrent.futures.as_completed()` (GH-3830) (#3831)
https://github.com/python/cpython/commit/2e3fd03796528a9f88f79d38e1d890af153c8e61

Cherry picked into 3.6.3 final -> closing

stage: patch review -> resolved