It's been absent from all alpha releases, which apparently nobody has noticed.
<!-- _freeze_importlib -->
<Projects Include="_freeze_importlib.vcxproj" />
<!-- Extension modules -->
- <ExtensionModules Include="_bz2;_ctypes;_decimal;_elementtree;_lzma;_msi;_multiprocessing;_overlapped;_sqlite3;_tkinter;pyexpat;select;unicodedata;winsound" />
+ <ExtensionModules Include="_bz2;_ctypes;_decimal;_elementtree;_lzma;_msi;_multiprocessing;_overlapped;_sqlite3;_tkinter;tix;pyexpat;select;unicodedata;winsound" />
<!-- _ssl will build _socket as well, which may cause conflicts in parallel builds -->
<ExtensionModules Include="_socket" Condition="!$(IncludeSSL)" />
<ExtensionModules Include="_ssl;_hashlib" Condition="$(IncludeSSL)" />