Build wheels for Linux/macOS, build sdist, and publish to PyPI

Also detect `linux/ipv6.h` by `_IPV6_H`, add push & pull request CI #5

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Run time

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Job	Run time
Build source distribution	9s
Build ubuntu-latest wheels	17m 10s
Build macos-latest wheels	3m 53s
Publish release to PyPI	0s
	21m 12s