This matches the documentation, and passes the test suite when multithreading
is disabled.

Patch by Anish Tambe.

when called from coroutines or callbacks.

Patch by Mariatta Wijaya.

Run expensive type checks only in debug mode.  In addition, stop
supporting passing handles to loop.run_in_executor.

the garbage collector is invoked in other thread.
Based on patch by Sebastian Cufre.

the garbage collector is invoked in other thread.
Based on patch by Sebastian Cufre.

Patch by Xiang Zhang.

Original patch by Mariatta Wijaya.

Patch by Mariatta Wijaya.

given empty data twice.  Patch by Benjamin Fogle.