MSI Python 3.15 GIL-enabled distributions don't include python3t.lib

[ngoldbaum]

Bug report

Bug description:

This is related to #148690 but was unfortunately not noticed then.

If I install 3.15 using using the MSI installer and don't check the "download free-threaded binaries" checkbox, I get a libs directory with the following content:

PS C:\Program Files\Python315> dir libs


    Directory: C:\Program Files\Python315\libs


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----          5/7/2026   2:59 PM         208036 python3.lib
-a----          5/7/2026   2:59 PM         451218 python315.lib
-a----          5/7/2026   2:59 PM           1750 _tkinter.lib

This means it's not possible to build abi3t extensions on Windows with this version of Python. Annoyingly, this is how github actions setup-python does it.

Note that the pymanager distribution and uv do include a python3t.lib file:

PS C:\Users\goldbaum\pyo3> dir C:\\Users\\goldbaum\\AppData\\Local\\Python\\pythoncore-3.15-64\\libs


    Directory: C:\Users\goldbaum\AppData\Local\Python\pythoncore-3.15-64\libs


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----         5/15/2026   9:43 AM         208036 python3.lib
-a----         5/15/2026   9:43 AM         451218 python315.lib
-a----         5/15/2026   9:43 AM         209010 python3t.lib

I'm pretty sure this was just an oversight but unfortunately fixing it will require changing the Windows file layout. Ping @zooba @hugovk.

Sorry for not catching this before beta1!

CPython versions tested on:

3.15

Operating systems tested on:

Windows

Linked PRs

• gh-149887: Install python3t.lib for GIL-enabled Windows install #149900

[zooba]

Should just need to be added to the dev_files.wxs file. It was certainly intended, so fixing it now is just a bugfix and there's no harm in doing that in b2. (I can't do this immediately, so if someone else wants to add a few entries and send a PR, go ahead)