Bump version for 0.15.6 release #265

Job	Run time
Lint & mypy	13s
Test & build (py3.12 on ubuntu-latest)	15s
Test & build (py3.11 on ubuntu-latest)	17s
Test & build (py3.10 on ubuntu-latest)	17s
Test & build (py3.9 on ubuntu-latest)	15s
Test & build (py3.8 on ubuntu-latest)	11s
Test & build (py3.7 on ubuntu-latest)	14s
Test & build (py3.12 on windows-latest)	50s
Test & build (py3.11 on windows-latest)	1m 9s
Test & build (py3.10 on windows-latest)	2m 17s
Test & build (py3.9 on windows-latest)	56s
Test & build (py3.8 on windows-latest)	44s
Test & build (py3.7 on windows-latest)	42s
Test & build (py3.12 on macos-13)	19s
Test & build (py3.11 on macos-13)	20s
Test & build (py3.10 on macos-13)	18s
Test & build (py3.9 on macos-13)	16s
Test & build (py3.8 on macos-13)	19s
Test & build (py3.7 on macos-13)	32s
	10m 24s

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