Hmm, I'm not seeing that behavior with either a freshly-built top of master 3.8 or with the python.org 3.7.1.

$ ./python  -c "import os;print(os.getgroups())"
[20, 12, 61, 79, 80, 81, 98, 33, 100, 204, 250, 395, 398, 399]
$ id -G
20 12 61 79 80 81 98 33 100 204 250 395 398 399
nad@harj:~/Projects/PyDev/active/dev/3x/source$ ./python -m test -w test_posix
Run tests sequentially
0:00:00 load avg: 1.64 [1/1] test_posix

== Tests result: SUCCESS ==

1 test OK.

Total duration: 13 sec 938 ms
Tests result: SUCCESS
$ ./python
Python 3.8.0a0 (heads/master:9e95eb0d60, Oct 25 2018, 15:55:56)
[Clang 10.0.0 (clang-1000.11.45.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ^D