bpo-40421: Add PyFrame_GetCode() function (GH-19757)

PyFrame_GetCode(frame): return a borrowed reference to the frame
code.

Replace frame->f_code with PyFrame_GetCode(frame) in most code,
except in frameobject.c, genobject.c and ceval.c.

Also add PyFrame_GetLineNumber() to the limited C API.
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 435808f..afde3db 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1074,8 +1074,10 @@
 
 .. c:function:: PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate)
 
-   Get the current frame of the Python thread state *tstate*. It can be
-   ``NULL`` if no frame is currently executing.
+   Get a borrowed reference to the current frame of the Python thread state
+   *tstate*.
+
+   Return ``NULL`` if no frame is currently executing.
 
    See also :c:func:`PyEval_GetFrame`.