Python as_completed failed on gRPC futures

[yutkin]

What version of gRPC and what language are you using?

Python 3.6.8
grpcio 1.19.0
grpcio-tools 1.18.0

What operating system (Linux, Windows,...) and version?

Linux

What did you do?

Using Python concurrent.futures.as_completed for gRPC futures.

What did you see?

...
> File "service.py", line 284, in Handle
   for i in futures.as_completed(ff, timeout=5):
            │       │            └ [<_Rendezvous of RPC that terminated with:
            │       │              	status = StatusCode.OK
            │       │              	details = ""
            │       │              >, <_Rendezvous object of in-flight RPC>, <_Ren...
            │       └ <function as_completed at 0x7f66f6c4e268>
            └ <module 'concurrent.futures' from '/usr/local/lib/python3.6/concurrent/futures/__init__.py'>
  File "/usr/local/lib/python3.6/concurrent/futures/_base.py", line 219, in as_completed
   with _AcquireFutures(fs):
        │               └ {<_Rendezvous object of in-flight RPC>, <_Rendezvous of RPC that terminated with:
        │                 	status = StatusCode.OK
        │                 	details = ""
        │                 >, <_Ren...
        └ <class 'concurrent.futures._base._AcquireFutures'>
 File "/usr/local/lib/python3.6/concurrent/futures/_base.py", line 146, in __enter__
   future._condition.acquire()
   └ <_Rendezvous object of in-flight RPC>
AttributeError: '_Rendezvous' object has no attribute '_condition'```

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