Since PEP 562 is implemented, we can use lazy imports easily.
Asyncio uses concurrent.futures.Future and concurrent.futures.ThreadPoolExecutor,
but not concurrent.futures.ProcessPoolExecutor by default.
Since importing concurrent.futures.process is slow, I want to import it lazily.
And it make sense to import concurrent.futures.thread too, because it's very
easy when we start to use __getattr__ once.
Here is quick benchmark:
$ ./python -m perf command ./python -c 'import asyncio' # patched
.....................
command: Mean +- std dev: 60.5 ms +- 0.1 ms
$ git stash
$ ./python -m perf command ./python -c 'import asyncio' # original
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command: Mean +- std dev: 74.3 ms +- 0.2 ms