I've been debugging this and I can repro on El Capitan on a different machine as well -- it's an infrequent hang, I've been running in a loop:

mattb@mattb-mbp:~/src/misc/cpython master$ for i in $(seq 1000); do echo "Run: $i -- $(date)"; time./python.exe -m test --timeout 180 test_multiprocessing_forkserver || break; sleep 10; done

And in this case after ~30 successful runs:

Run: 31 -- Tue Jun 20 05:12:19 PDT 2017
Run tests sequentially
0:00:00 load avg: 4.28 [1/1] test_multiprocessing_forkserver
Timeout (0:03:00)!
Thread 0x00007fffa1b3a3c0 (most recent call first):
  File "/Users/mattb/src/misc/cpython/Lib/selectors.py", line 415 in select
  File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/connection.py", line 916 in wait
  File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/popen_fork.py", line 45 in wait
  File "/Users/mattb/src/misc/cpython/Lib/multiprocessing/process.py", line 121 in join
  File "/Users/mattb/src/misc/cpython/Lib/test/_test_multiprocessing.py", line 431 in test_many_processes
  File "/Users/mattb/src/misc/cpython/Lib/unittest/case.py", line 605 in run
  File "/Users/mattb/src/misc/cpython/Lib/unittest/case.py", line 653 in __call__
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 122 in run
  File "/Users/mattb/src/misc/cpython/Lib/unittest/suite.py", line 84 in __call__
  File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1772 in run
  File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1896 in _run_suite
  File "/Users/mattb/src/misc/cpython/Lib/test/support/__init__.py", line 1936 in run_unittest
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 168 in test_runner
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 169 in runtest_inner
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/runtest.py", line 137 in runtest
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 374 in run_tests_sequential
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 454 in run_tests
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 530 in _main
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 504 in main
  File "/Users/mattb/src/misc/cpython/Lib/test/libregrtest/main.py", line 573 in main
  File "/Users/mattb/src/misc/cpython/Lib/test/__main__.py", line 2 in <module>
  File "/Users/mattb/src/misc/cpython/Lib/runpy.py", line 85 in _run_code
  File "/Users/mattb/src/misc/cpython/Lib/runpy.py", line 193 in _run_module_as_main

real    3m0.115s
user    0m5.804s
sys     0m0.683s

I also ran ~600 passes overnight on this test on an Ubuntu 16.04 machine without a hang, so it seems to be OSX specific.