Explicit documentation of which version of concurrent.futures is backported

Given the version number I assume this is a backport of concurrent.futures from Python 3.2. There have been improvements to the module in later releases of Python 3, and I was wondering if there are any plans to pull them in? I'm specifically interested in improvements to [ProcessPoolExecutor](https://docs.python.org/3.7/library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor), namely

1. the improved behavior on abrupt worker termination (3.3)
2. initializer / initargs support (3.7)

Thanks!