• 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