Use macro versions instead of function versions when we already know the type.

This will hopefully get rid of some Coverity warnings, be a hint to
developers, and be marginally faster.

Some asserts were added when the type is currently known, but depends
on values from another function.

Python/traceback.c

@@ -165,7 +165,7 @@ tb_displayline(PyObject *f, char *filename, int lineno, char *name)
 				}
 				if (PyString_Check(v)) {
 					size_t len;
-					len = PyString_Size(v);
+					len = PyString_GET_SIZE(v);
 					if (len + 1 + taillen >= MAXPATHLEN)
 						continue; /* Too long */
 					strcpy(namebuf, PyString_AsString(v));