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.