Intermittent/sporadic issues with PyPy?

[eriknw]

Following up on #169 (comment)

We have seen test failures when using wheels of these builds:

• pp37-manylinux_aarch64
  • Segfault in test_dicttoolz.py:test_merge_with
• pp38-win_amd64

Error from conda-forge

• linux_ppc64le_python3.8.____73_pypy
  • Segfault in test_merge_with

These were seen during automated wheel building via cibuildwheel in Github Actions. I don't know how important cytoolz on PyPy is, but it would be nice to understand and fix these errors.