[MilesCranmerBot]

Supports CPython free-threaded builds (ABI tag "t", e.g. python3.14t) by:

• recording whether the loaded libpython is free-threaded (via Py_GetVersion() string)
• adding parallel FT header structs (PyObjectFT/PyVarObjectFT/etc) while keeping existing PyObject layouts
• avoiding direct PyTypeObject field peeks in hot paths: use PyType_GetFlags + PyType_GetSlot
• fixing Juliacall heap type layout/alloc/free to respect the FT header size

Local testing (Julia 1.10.10):

• Smoke: pyimport("sys").version works for python 3.13.11 and python 3.14.2t
• Pkg.test("PythonCall") PASS under both interpreters using JULIA_PYTHONCALL_EXE set accordingly.

Commit: 5783d27