ptvsd_subprocess is not received on linux with py2.7

[karthiknadig]

Environment data

• PTVSD version: master
• OS and version: ubuntu 16.04
• Python version (& distribution if applicable, e.g. Anaconda): 2.7
• Using VS Code or Visual Studio: console tests
• Package: Flask

Actual behavior

Cannot debug Flask subprocess on linux.

Expected behavior

Should be able to debug Flask subprocess on linux with python 2.7

Steps to reproduce:

Clone ptvsd the following test:

python -m pytest -vvs ./pytests/func/test_flask.py::test_flask_breakpoint_multiproc[launch]

Log

@00.250000: ptvsd#60053 ERR  * Restarting with stat
@00.250000: ptvsd#60053 ERR Traceback (most recent call last):
@00.250000: ptvsd#60053 ERR
@00.343750: ptvsd#60053 ERR Exception in thread ptvsd.EventLoop (most likely raised during interpreter shutdown):Killed

[karthiknadig]

Note this can be reproed with subprocess.Popen on linux py2.7

[karthiknadig]

Re-enable following tests for linux py 2.7 after this bug is fixed:

test_flask.py::test_flask_breakpoint_multiproc
test_multiproc.py::test_subprocess

[int19h]

The short story is that subprocess.Popen uses fork() on Unix in Python 2.7. So this is basically a dupe of #943.