Select Python version in Cygwin

[radarhere]

Cygwin started failing in main, with https://github.com/python-pillow/Pillow/actions/runs/4696586061/jobs/8329527067#step:8:157

  gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -DFFI_BUILDING=1 -DUSE__THREAD -DHAVE_SYNC_SYNCHRONIZE -I/usr/include/ffi -I/usr/include/libffi -I/usr/include/python3.9 -c c/_cffi_backend.c -o build/temp.cygwin-3.4.6-x86_64-cpython-39/c/_cffi_backend.o
  c/_cffi_backend.c:2:10: fatal error: Python.h: No such file or directory
     2 | #include <Python.h>
       |          ^~~~~~~~~~

The above lines mention Python 3.9, but this should be a Python 3.8 build.

This PR links python3 to the correct Python version.

However, problems with ImageMagick started happening a day later, so the Cygwin builds are still failing.

[hugovk]

Thanks, let's merge, it gets us a step closer back to green.

[radarhere]

For reasons unknown, the ImageMagick problem has now disappeared, so Cygwin is passing again.