gh-127271: Replace use of PyCell_GET/SET (gh-127272)

* 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.

1 files changed, 2 insertions, 0 deletions

diff --git a/Doc/howto/free-threading-extensions.rst b/Doc/howto/free-threading-extensions.rst
index 6abe93d..c1ad42e 100644
--- a/Doc/howto/free-threading-extensions.rst
+++ b/Doc/howto/free-threading-extensions.rst
@@ -167,6 +167,8 @@ that return :term:`strong references <strong reference>`.
 +-----------------------------------+-----------------------------------+
 | :c:func:`PyImport_AddModule`      | :c:func:`PyImport_AddModuleRef`   |
 +-----------------------------------+-----------------------------------+
+| :c:func:`PyCell_GET`              | :c:func:`PyCell_Get`              |
++-----------------------------------+-----------------------------------+
 
 Not all APIs that return borrowed references are problematic.  For
 example, :c:func:`PyTuple_GetItem` is safe because tuples are immutable.