PYTHON-4731 - Explicitly close all MongoClients opened during tests #563

Job	Run time
Make SDist	38s
Build wheels for manylinux_x86_64	2m 35s
Build wheels for manylinux_aarch64	10m 18s
Build wheels for manylinux_ppc64le	9m 48s
Build wheels for manylinux_s390x	10m 20s
Build wheels for manylinux_i686	2m 28s
Build wheels for win_amd6	4m 43s
Build wheels for win32	5m 39s
Build wheels for macos	3m 35s
Download Wheels	7s
	50m 11s

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