bpo-40060 Expose socket.TCP_NOTSENT_LOWAT on macOS in official builds

[dimaqq]

Somehow, it turns out that TCP_NOTSENT_LOWAT that's available since 3.7.x is not available in the official macOS builds 🙀
This change exports the constant even if cpython is built against very old header files, as it's presumed to be the case for official macOS builds.
https://bugs.python.org/issue40060

https://bugs.python.org/issue40060

[dimaqq]

I'm not sure why Azure Pipeline failed, and now it seems the build log was cleaned up and I can't see it.
What can I do to move this forward?

[dimaqq]

The job running on runner GitHub Actions 3 has exceeded the maximum execution time of 360.

What now?

[dimaqq]

The logs goes like:

0:11:10 load avg: 6.42 [421/423] test_importlib passed 0:11:40 load avg: 5.40 running: test_subprocess (58.5 sec), test_multiprocessing_spawn (53.0 sec) 0:12:10 load avg: 5.48 running: test_subprocess (1 min 28 sec), test_multiprocessing_spawn (1 min 23 sec) 0:12:40 load avg: 5.44 running: test_subprocess (1 min 58 sec), test_multiprocessing_spawn (1 min 53 sec) 0:12:57 load avg: 4.58 [422/423] test_multiprocessing_spawn passed (2 min 10 sec) -- running: test_subprocess (2 min 16 sec) 0:13:27 load avg: 2.89 running: test_subprocess (2 min 46 sec) 0:13:57 load avg: 2.11 running: test_subprocess (3 min 16 sec) 0:14:27 load avg: 1.54 running: test_subprocess (3 min 46 sec) ... 5:53:33 load avg: 0.48 running: test_subprocess (5 hour 42 min) 5:54:03 load avg: 0.51 running: test_subprocess (5 hour 43 min) 5:54:33 load avg: 0.31 running: test_subprocess (5 hour 43 min) Terminate batch job (Y/N)? 

So, not my fault :)

[terryjreedy]

Re-running after Tests/Windows(x64) failed when test_subprocessing hung until timeout. All other tests in that run and all other runs passed. The same test passed on my machine on master and PR branch with re-compile. Re-running here from the error page failed after a minute. Going so far after 10 minutes after re-open.

[dimaqq]

@terryjreedy feel free to use this PR to debug the build thing if you want :)