xmss: small touchup #1002

Job	Run time
Lint and format checks - Python 3.12	2m 5s
Lint and format checks - Python 3.13	2m 16s
Tests - Python 3.14 on ubuntu-latest	2m 36s
Tests - Python 3.12 on macos-latest	3m 57s
Tests - Python 3.12 on ubuntu-latest	3m 20s
Tests - Python 3.13 on ubuntu-latest	3m 19s
Lint and format checks - Python 3.14	2m 5s
Tests - Python 3.13 on macos-latest	4m 36s
Tests - Python 3.14 on macos-latest	3m 51s
Fill test fixtures - Python 3.14	3m 47s
	31m 52s

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