Python 3.12: Fix building Pillow on Windows

[hugovk]

For #6941

Pillow doesn't build for Windows on 3.12:

diff --git a/.github/workflows/test-windows.yml b/.github/workflows/test-windows.yml
index e938e999d..306e34ca9 100644
--- a/.github/workflows/test-windows.yml
+++ b/.github/workflows/test-windows.yml
@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12-dev"]
         architecture: ["x86", "x64"]
         include:
           # PyPy 7.3.4+ only ships 64-bit binaries for Windows

Python 3.12-dev x86 fails:

   Creating library build\temp.win32-cpython-312\Release\src\_imaging.cp312-win32.lib and object build\temp.win32-cpython-312\Release\src\_imaging.cp312-win32.exp
display.obj : error LNK2001: unresolved external symbol _PyUnicode_AS_UNICODE
build\lib.win32-cpython-312\PIL\_imaging.cp312-win32.pyd : fatal error LNK1120: 1 unresolved externals
error: command 'C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.34.31933\\bin\\HostX86\\x86\\link.exe' failed with exit code 1120

Python 3.12-dev x64 similarly fails:

   Creating library build\temp.win-amd64-cpython-312\Release\src\_imaging.cp312-win_amd64.lib and object build\temp.win-amd64-cpython-312\Release\src\_imaging.cp312-win_amd64.exp
display.obj : error LNK2001: unresolved external symbol PyUnicode_AS_UNICODE
build\lib.win-amd64-cpython-312\PIL\_imaging.cp312-win_amd64.pyd : fatal error LNK1120: 1 unresolved externals
error: command 'C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.34.31933\\bin\\HostX86\\x64\\link.exe' failed with exit code 1120

This is because PyUnicode_AS_UNICODE() was deprecated in Python 3.3 and removed in 3.12:

• https://docs.python.org/3.12/whatsnew/3.12.html?highlight=pyunicode_as_unicode#id5
• PEP 623
• https://docs.python.org/3/c-api/unicode.html?highlight=pyunicode_as_unicode#c.PyUnicode_AS_UNICODE

Changed in version 3.3: This function is now inefficient – because in many cases the Py_UNICODE representation does not exist and needs to be created – and can fail (return NULL with an exception set). Try to port the code to use the new PyUnicode_nBYTE_DATA() macros or use PyUnicode_WRITE() or PyUnicode_READ().

Deprecated since version 3.3, will be removed in version 3.12: Part of the old-style Unicode API, please migrate to using the PyUnicode_nBYTE_DATA() family of macros.

[nulano]

The macro is used in list_windows_callback, which is referenced from PyImaging_ListWindowsWin32, i.e. Image.core.listwindows. That is an internal function that I can't see any references to, so I think it should be safe to just remove it.

Pillow/src/display.c

Line 438 in bc1a96a

GetWindowTextW(hwnd, PyUnicode_AS_UNICODE(title), title_size + 1);

Pillow/src/display.c

Line 487 in bc1a96a

EnumWindows(list_windows_callback, (LPARAM)window_list);

Pillow/src/_imaging.c

Line 4072 in bc1a96a

{"listwindows", (PyCFunction)PyImaging_ListWindowsWin32, METH_VARARGS},

I think this was intended to be used with ImageGrab.grab(bbox=Image.core.listwindows[...][...]) (i.e. what #4415 (comment) asked for), but it would not help with that issue as applications aren't always visible on the desktop.

[hugovk]

Thanks, I've updated #6943.