I was looking into this and it seems that using replacing PendingDeprecationWarning with DeprecationWarning fixes the issue. Should I be using check_warnings?

Attaching a patch of this so that I can raise a PR for the same.

I grepped for PendingDeprecationWarning and there is a similar crash with test_ssl

➜  cpython git:(1bf9cc5) ✗ ./python -Werror -m test -v test_ssl
== CPython 3.7.0 (v3.7.0:1bf9cc5, Jun 28 2018, 11:20:40) [GCC 5.4.0 20160609]
== Linux-4.4.0-127-generic-x86_64-with-debian-stretch-sid little-endian
== cwd: /home/cpython/build/test_python_31365
== CPU count: 1
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.08 [1/1] test_ssl
test test_ssl crashed -- Traceback (most recent call last):
  File "/home/cpython/Lib/test/libregrtest/runtest.py", line 176, in runtest_inner
    test_runner()
  File "/home/cpython/Lib/test/test_ssl.py", line 4151, in test_main
    plat = func()
  File "/home/cpython/Lib/platform.py", line 305, in linux_distribution
    "in Python 3.5", DeprecationWarning, stacklevel=2)
DeprecationWarning: dist() and linux_distribution() functions are deprecated in Python 3.5

test_ssl failed

== Tests result: FAILURE ==

1 test failed:
    test_ssl

Total duration: 194 ms
Tests result: FAILURE

Thanks