Fix test_wantWriteError failure on macOS #1394

Job	Run time
Downstream tests for twisted	1m 54s
Downstream tests for certbot	34s
tests-cryptography-main on ubuntu-rolling	1m 53s
Downstream tests for certbot-josepy	30s
linux (3.12, tests)	1m 3s
linux (3.9, tests)	1m 20s
linux (3.11, tests-cryptography-main)	1m 47s
linux (3.10, tests)	1m 11s
linux (3.14, tests)	1m 10s
linux (3.10, tests-cryptography-main)	1m 53s
linux (3.9, tests-cryptography-minimum)	1m 13s
linux (3.14t, tests-wheel, windows-latest)	1m 37s
linux (3.8, tests)	1m 10s
linux (pypy-3.11, tests-cryptography-main)	2m 27s
linux (3.10, tests-cryptography-minimum)	1m 12s
linux (3.13, tests)	1m 9s
linux (3.13, tests-cryptography-main)	1m 51s
linux (3.9, docs)	30s
linux (3.12, tests-cryptography-main)	1m 49s
linux (3.8, tests-cryptography-main)	1m 58s
linux (3.9, tests-cryptography-main)	1m 58s
linux (3.14t, tests-cryptography-main)	2m 0s
linux (3.11, tests-wheel, windows-latest)	1m 3s
linux (pypy-3.11, tests-cryptography-minimum)	1m 35s
linux (pypy-3.11, tests)	1m 39s
linux (3.11, lint)	10s
linux (3.14, tests-cryptography-main)	1m 57s
linux (3.8, tests-cryptography-minimum)	1m 10s
linux (3.9, tests-cryptography-minimum-wheel)	31s
linux (3.11, tests-cryptography-minimum)	1m 2s
linux (3.11, tests)	1m 10s
linux (3.14t, tests)	1m 18s
linux (3.9, tests-wheel)	31s
linux (3.12, tests-cryptography-minimum)	1m 15s
linux (3.13, tests-cryptography-minimum)	1m 11s
linux (3.9, tests-random-order)	1m 17s
linux (3.13, mypy)	22s
linux (3.9, check-manifest)	18s
all-green	10s
	48m 48s

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