* Replace uses of `PyCell_GET` and `PyCell_SET`. These macros are not
safe to use in the free-threaded build. Use `PyCell_GetRef()` and
`PyCell_SetTakeRef()` instead.
* Since `PyCell_GetRef()` returns a strong rather than borrowed ref, some
code restructuring was required, e.g. `frame_get_var()` returns a strong
ref now.
* Add critical sections to `PyCell_GET` and `PyCell_SET`.
* Move critical_section.h earlier in the Python.h file.
* Add `PyCell_GET` to the free-threading howto table of APIs that return
borrowed refs.
* Add additional unit tests for free-threading.
Avoid mixing declarations and code in the C API to fix the compiler
warning: "ISO C90 forbids mixed declarations and code"
[-Werror=declaration-after-statement].
Convert the following macros to static inline functions:
* PyCell_GET()
* PyCell_SET()
Limited C API version 3.12 no longer casts arguments.
Fix also usage of PyCell_SET(): only delete the old value after
setting the new value.
Add a new _Py_RVALUE() macro to prevent using an expression as an
l-value.
Replace a "(void)" cast with the _Py_RVALUE() macro in the following
macros:
* PyCell_SET()
* PyList_SET_ITEM()
* PyTuple_SET_ITEM()
* _PyGCHead_SET_FINALIZED()
* _PyGCHead_SET_NEXT()
* asdl_seq_SET()
* asdl_seq_SET_UNTYPED()
Add also parentheses around macro arguments in PyCell_SET() and
PyTuple_SET_ITEM() macros.
