Python 3.8 + Multiprocessing

[effigies]

In #3154, several tests had to be marked skip, xfail or timeout to deal with some new quirk of multiprocessing. It seems likely to be a combination of the following:

• Python 3.8 multiprocessing module
• pytest subprocesses (xdist)
• Nipype configurations for setting up concurrent.futures and multiprocessing in the MultiProc/LegacyMultiProc plugins

[effigies]

Also, nipype/pipeline/plugins/tests/test_callback.py::test_callback_exception[False-LegacyMultiProc] times out in https://travis-ci.org/github/nipy/nipype/jobs/679493619.

Should mark with 60s timeout or other sensible value.

Edit: Made the mistake of restarting, and Travis doesn't register a new job ID for reruns... it may not show an error