Broken tests on macOS

We are having problems (NixOS/nixpkgs#76197) with the following three tests:

* test_subprocess_print
* test_subprocess_error
* test_ipython_start_kernel_no_userns - (this one broke only when we updated jedi from 0.14.1 to 0.15.1)

All three tests produce a crash:
```
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY

Application Specific Information:
abort() called

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff649e22c6 __pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fff64a9dbf1 pthread_kill + 284
2 libsystem_c.dylib 0x00007fff648ffd8a raise + 26
3 libsystem_platform.dylib 0x00007fff64a92b5d _sigtramp + 29
4 ??? 000000000000000000 0 + 0
5 libsystem_c.dylib 0x00007fff6494c6a6 abort + 127
6 libzmq.5.dylib 0x0000000111627a19 zmq::zmq_abort(char const*) + 9
7 libzmq.5.dylib 0x000000011162b13a zmq::kqueue_t::kqueue_t(zmq::thread_ctx_t const&) + 170
8 libzmq.5.dylib 0x00000001116283ca zmq::io_thread_t::io_thread_t(zmq::ctx_t*, unsigned int) + 106
9 libzmq.5.dylib 0x000000011161d03b zmq::ctx_t::start() + 555
10 libzmq.5.dylib 0x000000011161b9b5 zmq::ctx_t::create_socket(int) + 485
11 libzmq.5.dylib 0x0000000111665d46 zmq_socket + 38
12 socket.cpython-37m-darwin.so 0x00000001116fcd42 __pyx_pw_3zmq_7backend_6cython_6socket_6Socket_1__init__ + 1922
13 libpython3.7m.dylib 0x000000010e2b799c wrap_init + 12
14 libpython3.7m.dylib 0x000000010e25f7b8 PyObject_Call + 136
15 libpython3.7m.dylib 0x000000010e337b8f _PyEval_EvalFrameDefault + 29967
16 libpython3.7m.dylib 0x000000010e33ba95 _PyEval_EvalCodeWithName + 2981
17 libpython3.7m.dylib 0x000000010e25f125 _PyFunction_FastCallDict + 501
18 libpython3.7m.dylib 0x000000010e260766 _PyObject_Call_Prepend + 150
19 libpython3.7m.dylib 0x000000010e2b78f1 slot_tp_init + 177
20 libpython3.7m.dylib 0x000000010e2b2de4 type_call + 292
21 libpython3.7m.dylib 0x000000010e25f7b8 PyObject_Call + 136
```

Also when I run specifically those tests (e.g. `pytest -k test_subprocess_print`) the tests pass. However when they are run together only these three are the failure points. This part I don't understand, because my understanding is that pytest runs those sequentially. I suspect that the subprocesses created in those tests may be lingering and interfering with each other.

Any pointers would be much appreciated.

Some package versions:
* ipykernel 5.1.2
* jupyter_client 5.3.4
* jupyter_core 4.6.0
* ipython 7.8.0
* pyzmq 18.1.0
* tornado 5.1