test

Add the 'btree' benchmark. #1820

Job	Run time
ubuntu-latest - 3.9	6m 31s
ubuntu-latest - 3.10	6m 35s
ubuntu-latest - 3.11	7m 34s
ubuntu-latest - 3.12	8m 7s
ubuntu-latest - 3.13	7m 25s
ubuntu-24.04-arm - 3.9	5m 38s
ubuntu-24.04-arm - 3.10	5m 48s
ubuntu-24.04-arm - 3.11	7s
ubuntu-24.04-arm - 3.12	6m 51s
ubuntu-24.04-arm - 3.13	6m 44s
ubuntu-latest - 3.13 free-threading	7m 57s
ubuntu-24.04-arm - 3.13 free-threading	6m 40s
ubuntu-latest - 3.14-dev	7m 48s
ubuntu-latest - 3.14-dev free-threading	8m 2s
ubuntu-latest - pypy-3.10	10m 20s
ubuntu-latest - pypy-3.9	10m 7s
macos-latest - 3.13	6m 42s
windows-latest - 3.13	12m 40s
	2h 11m 36s

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