Add a global variable to set number of parallel threads #496

Job	Run time
source check	45s
wheels for ubuntu-latest / x86_64	45s
sdist on ubuntu-latest with py 3.8	48s
wheels for ubuntu-latest / i686	47s
sdist on ubuntu-latest with py 3.9	46s
wheels for windows-latest / AMD64	31s
sdist on ubuntu-latest with py 3.10	44s
wheels for windows-latest / x86	33s
sdist on ubuntu-latest with py 3.11	48s
wheels for macos-latest / x86_64	1s
sdist on ubuntu-latest with py 3.12	46s
wheels for macos-latest / arm64	1s
wheels for macos-latest / universal2	1s
sdist on windows-latest with py 3.8	35s
sdist on windows-latest with py 3.9	34s
sdist on windows-latest with py 3.10	35s
sdist on windows-latest with py 3.11	34s
sdist on windows-latest with py 3.12	34s
sdist on macos-latest with py 3.8	47s
sdist on macos-latest with py 3.9	47s
sdist on macos-latest with py 3.10	46s
sdist on macos-latest with py 3.11	48s
sdist on macos-latest with py 3.12	47s
upload_to_pypi	1s
	14m 4s

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