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/ceval.h b/Include/cpython/ceval.h
index 020f787..e1922a6 100644
--- a/Include/cpython/ceval.h
+++ b/Include/cpython/ceval.h
@@ -23,7 +23,7 @@
    flag was set, else return 0. */
 PyAPI_FUNC(int) PyEval_MergeCompilerFlags(PyCompilerFlags *cf);
 
-PyAPI_FUNC(PyObject *) _PyEval_EvalFrameDefault(PyThreadState *tstate, struct _frame *f, int exc);
+PyAPI_FUNC(PyObject *) _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int exc);
 
 PyAPI_FUNC(void) _PyEval_SetSwitchInterval(unsigned long microseconds);
 PyAPI_FUNC(unsigned long) _PyEval_GetSwitchInterval(void);