Trio-parallel

- Changed subprocess context to explicitly always spawn new processes (5)
- Changed synchronization scheme to achieve full passing tests on

- Windows, Linux, MacOS
- CPython 3.6, 3.7, 3.8, 3.9
- Pypy 3.6, 3.7, 3.7-nightly

Note Pypy on Windows is not supported here or by Trio (10)

Exported from https://github.com/python-trio/trio/pull/1783