bpo-21302: Add nanosleep() implementation for time.sleep() in Unix

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https://bugs.python.org/issue21302

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I created this PR based on @Livius90's PR #28526 to propose a "Refactor" commit.

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The PR #28526 LGTM, but I propose to write the #ifdef dance differently and rename secs to timeout to make the code more readable. So I created this PR based on PR #28526. For the main code in Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS, I merged the 3 code paths to put if #ifdef in the Py_BEGIN_ALLOW_THREADS block.

@Livius90: Is it better? What do you think?

[Livius90]

I can not say that the new Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS #ifdef section is better and more readable. I lost to see easily these have different err variables passing/checking. Any other new naming style is better, sure.

https://github.com/python/cpython/blob/d7a6cdd1711c5e55e81c37cf4acc173d9093d83e/Modules/timemodule.c#L2092

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I lost to see easily these have different err variables passing/checking.

Ok, I moved err closer to the function call.

[Livius90]

I think, It is much better, thanks.

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Thanks @Livius90! I merged the PR based on your PR :-) I also updated the documentation.