Most importantly, this resolves the issues with functions and types defined in __main__.
It also expands the number of supported objects and simplifies the implementation.
PEP-734 has been accepted (for 3.14).
(FTR, I'm opposed to putting this under the concurrent package, but
doing so is the SC condition under which the module can land in 3.14.)
This is an implementation of InterpreterPoolExecutor that builds on ThreadPoolExecutor.
(Note that this is not tied to PEP 734, which is strictly about adding a new stdlib module.)
Possible future improvements:
* support passing a script for the initializer or to submit()
* support passing (most) arbitrary functions without pickling
* support passing closures
* optionally exec functions against __main__ instead of the their original module
