PYTHON-4731 - Explicitly close all MongoClients opened during tests #586

Job	Run time
Make SDist	40s
Build wheels for manylinux_x86_64	2m 28s
Build wheels for manylinux_aarch64	10m 11s
Build wheels for manylinux_ppc64le	9m 44s
Build wheels for manylinux_s390x	10m 2s
Build wheels for manylinux_i686	2m 26s
Build wheels for win_amd6	4m 53s
Build wheels for win32	5m 14s
Build wheels for macos	3m 26s
Download Wheels	9s
	49m 13s

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