Cygwin tests omitted on Windows (and vice versa)

Cygwin Python tests are omitted on Windows (and vice versa). The documentation for xdist says:

Multi-Platform coverage: you can specify different Python interpreters or different platforms and run tests in parallel on all of them.

============================= test session starts =============================
platform win32 -- Python 3.6.0a4, pytest-3.0.0, py-1.4.31, pluggy-0.3.1 -- f:\portableapps\python3x\python.exe
cachedir: .cache
rootdir: F:\cygwin\home\thorsten\python\modules, inifile: pytest.ini
plugins: xdist-1.15.0
[gw0] win32 Python 2.7.12 cwd: F:\cygwin\home\thorsten\python\modules
[gw1] win32 Python 3.6.0 cwd: F:\cygwin\home\thorsten\python\modules
[gw2] cygwin Python 2.7.10 cwd: /home/thorsten/python/modules
[gw3] cygwin Python 3.4.3 cwd: /home/thorsten/python/modules
[gw0] Python 2.7.12 (v2.7.12:d33e0cf91556, Jun 27 2016, 15:19:22) [MSC v.1500 32 bit (Intel)]
[gw1] Python 3.6.0a4 (v3.6.0a4:017cf260936b, Aug 16 2016, 00:59:16) [MSC v.1900 64 bit (AMD64)]
[gw3] Python 3.4.3 (default, May  5 2015, 17:58:45)  -- [GCC 4.9.2]
[gw2] Python 2.7.10 (default, Jun  1 2015, 18:05:38)  -- [GCC 4.9.2]
gw0 [7] / gw1 [7] / gw2 [0] / gw3 [0]

The equivalent on Cygwin:

================================================================= test session starts =================================================================platform cygwin -- Python 3.4.3, pytest-3.0.0, py-1.4.31, pluggy-0.3.1 -- /usr/bin/python3
cachedir: .cache
rootdir: /home/thorsten/python/modules, inifile: pytest.ini
plugins: xdist-1.15.0
[gw0] win32 Python 2.7.12 cwd: F:\cygwin\home\thorsten\python\modules
[gw1] win32 Python 3.6.0 cwd: F:\cygwin\home\thorsten\python\modules
[gw2] cygwin Python 2.7.10 cwd: /home/thorsten/python/modules
[gw3] cygwin Python 3.4.3 cwd: /home/thorsten/python/modules
[gw0] Python 2.7.12 (v2.7.12:d33e0cf91556, Jun 27 2016, 15:19:22) [MSC v.1500 32 bit (Intel)]
[gw1] Python 3.6.0a4 (v3.6.0a4:017cf260936b, Aug 16 2016, 00:59:16) [MSC v.1900 64 bit (AMD64)]
[gw3] Python 3.4.3 (default, May  5 2015, 17:58:45)  -- [GCC 4.9.2]
[gw2] Python 2.7.10 (default, Jun  1 2015, 18:05:38)  -- [GCC 4.9.2]
gw0 [0] / gw1 [0] / gw2 [7] / gw3 [7]

My pytest.ini

[pytest]
# https://pytest.org/latest/doctest.html
doctest_optionflags = ALLOW_UNICODE

# syntax for `--tx`: http://codespeak.net/execnet/basics.html
addopts             = --doctest-modules
                          --tx popen//python=F:/PortableApps/Python2x/python.exe
                          --tx popen//python=F:/PortableApps/Python3x/python.exe
                          --tx popen//python=F:/cygwin/bin/python2.7.exe
                          --tx popen//python=F:/cygwin/bin/python3.4m.exe
                          --dist=each

Start with the pytest.ini configuration and compare the two reported test-session outputs, especially the --tx interpreters and --dist=each setting. Reproduce the run on Windows and Cygwin, then verify that each platform's workers execute the expected tests rather than reporting zero tests.