[issue20130] asyncio: implement a synchronous executor if concurrent.futures is missing

[STINNER Victor]

STINNER Victor added the comment:

> https://bitbucket.org/haypo/trollius/src/8f28756d63c2c9f09c3a7c1df420796eca9ff08b/asyncio/executor.py?at=trollius#cl-60

Oh, there was a forgotten "raise" instruction. I added it to test the
exception case, but it looks like this case is not tested by asyncio
test suite. (I just removed the "raise" instruction.)

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