gh-108765: Python.h no longer includes <unistd.h> by vstinner · Pull Request #108783 · python/cpython

This fixes the following compilation error with Python 3.13.0a1:
```
c-ext/backend_c.c:275:13: error: implicit declaration of function ‘sysconf’; did you mean ‘sscanf’? [-Werror=implicit-function-declaration]
  275 |     count = sysconf(_SC_NPROCESSORS_ONLN);
      |             ^~~~~~~
      |             sscanf
```

According to https://docs.python.org/3.13/whatsnew/3.13.html:

Python.h no longer includes the <unistd.h> standard header file.
If needed, it should now be included explicitly.
For example, it provides the functions: read(), write(), close(), isatty(), lseek(), getpid(), getcwd(), sysconf() and getpagesize().
As a consequence, _POSIX_SEMAPHORES and _POSIX_THREADS macros are no longer defined by Python.h.
The HAVE_UNISTD_H and HAVE_PTHREAD_H macros defined by Python.h can be used to decide if <unistd.h> and <pthread.h> header files can be included.

See python/cpython#108783 .

This fixes the following compilation error with Python 3.13.0a1:
```
c-ext/backend_c.c:275:13: error: implicit declaration of function ‘sysconf’; did you mean ‘sscanf’? [-Werror=implicit-function-declaration]
  275 |     count = sysconf(_SC_NPROCESSORS_ONLN);
      |             ^~~~~~~
      |             sscanf
```

According to https://docs.python.org/3.13/whatsnew/3.13.html:

Python.h no longer includes the <unistd.h> standard header file.
If needed, it should now be included explicitly.
For example, it provides the functions: read(), write(), close(), isatty(), lseek(), getpid(), getcwd(), sysconf() and getpagesize().
As a consequence, _POSIX_SEMAPHORES and _POSIX_THREADS macros are no longer defined by Python.h.
The HAVE_UNISTD_H and HAVE_PTHREAD_H macros defined by Python.h can be used to decide if <unistd.h> and <pthread.h> header files can be included.

See python/cpython#108783 .