bpo-40421: Add pyframe.h header file (GH-19755)

Add a new separated pyframe.h header file of the PyFrame public C
API: it is included by Python.h.

Add PyFrame_GetLineNumber() to the limited C API.

Replace "struct _frame" with "PyFrameObject" in header files.
PyFrameObject is now defined as struct _frame by pyframe.h which is
included early enough in Python.h.
diff --git a/Include/cpython/traceback.h b/Include/cpython/traceback.h
index 746097d..837470c 100644
--- a/Include/cpython/traceback.h
+++ b/Include/cpython/traceback.h
@@ -9,7 +9,7 @@
 typedef struct _traceback {
     PyObject_HEAD
     struct _traceback *tb_next;
-    struct _frame *tb_frame;
+    PyFrameObject *tb_frame;
     int tb_lasti;
     int tb_lineno;
 } PyTracebackObject;