Run tests in parallel with pytest-xdist #1178

Job	Run time
Windows AMD64	34m 24s
Windows ARM64	14m 20s
sdist	24s
Windows x86	28m 5s
iOS arm64 device	8m 34s
macOS 10.15 x86_64	21m 2s
iOS x86_64 simulator	45m 33s
macOS 10.10 x86_64	18m 17s
manylinux_2_28 aarch64	15m 28s
manylinux_2_28 x86_64	19m 15s
manylinux2014 and musllinux x86_64	58m 33s
macOS arm64	9m 55s
macOS 10.13 x86_64	31m 48s
iOS arm64 simulator	25m 17s
manylinux2014 and musllinux aarch64	37m 37s
Count dists	0s
Upload release to PyPI	0s
Upload wheels to scientific-python-nightly-wheels	0s
	6h 8m 32s

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