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/classobject.c b/Objects/classobject.c
index 40cbeaa..4a9add1 100644
--- a/Objects/classobject.c
+++ b/Objects/classobject.c
@@ -375,7 +375,7 @@
 }
 
 void
-PyMethod_Fini(void)
+_PyMethod_Fini(void)
 {
     (void)PyMethod_ClearFreeList();
 }