Make PyXXX_Fini() functions private (GH-15531)

For example, rename PyTuple_Fini() to _PyTuple_Fini().

These functions are only declared in the internal C API.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index a796a59..27ef9ff 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -997,7 +997,7 @@
 }
 
 void
-PyFrame_Fini(void)
+_PyFrame_Fini(void)
 {
     (void)PyFrame_ClearFreeList();
 }