Fix for Python 3.13: explicitly include unistd.h

[hugovk]

Changes proposed in this pull request:

• Test Python 3.13 pre-release
• Explicitly include unistd.h to use lseek

Changed in Python 3.13:

• 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(), getpagesize(), alarm() and
  pause().
  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.

So we need to #include <unistd.h> for src/libImaging/TiffDecode.c which uses lseek.

---

Also add Python 3.13 to the CI for Ubuntu, macOS and Windows. We need to skip cffi, it's not ready for 3.13.

3.13 is in alpha, so we can expect things still to change. I'm fine leaving it out of the CI until later or beta, but it could be useful to get early feedback. If any sudden failures turn up, we can always comment it out until we get it sorted (in Pillow or upstream in CPython, as necessary).

[Yay295]

Isn't unistd.h Unix only? How is TiffDecode.c building on Windows?

[nulano]

On Windows, lseek is an alias for _lseek, both of which are declared in io.h which is included from Python.h via pyconfig.h.