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/floatobject.c b/Objects/floatobject.c
index 689e929..6643ff3 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -2031,7 +2031,7 @@
 }
 
 void
-PyFloat_Fini(void)
+_PyFloat_Fini(void)
 {
     (void)PyFloat_ClearFreeList();
 }