BUG: define __Pyx_PyUnstable_Object_IsUniqueReferencedTemporary for python<3.14

[Alvaro-Kothe]

• closes BUG: undefined symbol when building Pandas with a custom python installation #63128 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

---

Just want to point out that this definition is somewhat useless because just below this definition we have:

# Python version compatibility for PyUnstable_Object_IsUniqueReferencedTemporary
cdef inline bint _is_unique_referenced_temporary(object obj) except -1:
    if PY_VERSION_HEX >= 0x030E0000:
        # Python 3.14+ has PyUnstable_Object_IsUniqueReferencedTemporary
        return PyUnstable_Object_IsUniqueReferencedTemporary(obj)
    else:
        # Fallback for older Python versions using sys.getrefcount
        return sys.getrefcount(obj) <= 1

The new definition isn't used for Python<3.14, and uses the CPython definition for Python >=3.14. But at least the symbol is defined and won't cause import errors:

$ nm build/cp313d/pandas/_libs/internals.cpython-313d-x86_64-linux-gnu.so | grep Unique
0000000000000d00 t __Pyx_PyUnstable_Object_IsUniqueReferencedTemporary

[rhshadrach]

lgtm assuming green, cc @jorisvandenbossche

[jorisvandenbossche]

Can you add a comment here that this is unused and just added to avoid a build warning?

[Alvaro-Kothe]

Done.

I actually had to define it to prevent import errors in debug builds. When you build with optimizations, the compiler removes the declaration, but debug builds has this symbol undefined, which causes import errors.

[jorisvandenbossche]

Yes, this is certainly fine if that avoids the build warning