fix: flatten "base" in AssignmentEvent #111

Job	Run time
linux (ubuntu-latest, x86_64)	58s
linux (ubuntu-latest, x86)	1m 0s
linux (ubuntu-latest, aarch64)	1m 1s
macos (macos-12, x86_64)	17s
musllinux (ubuntu-latest, x86_64)	40s
macos (macos-14, aarch64)	14s
linux (ubuntu-latest, armv7)	1m 3s
musllinux (ubuntu-latest, x86)	42s
linux (ubuntu-latest, s390x)	1m 1s
linux (ubuntu-latest, ppc64le)	1m 1s
musllinux (ubuntu-latest, aarch64)	38s
sdist	10s
musllinux (ubuntu-latest, armv7)	39s
windows (windows-latest, x64)	45s
windows (windows-latest, x86)	41s
Release	0s
	10m 50s

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