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/dictobject.c b/Objects/dictobject.c
index fec3a87..73956df 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -282,7 +282,7 @@
 
 
 void
-PyDict_Fini(void)
+_PyDict_Fini(void)
 {
     PyDict_ClearFreeList();
 }