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/methodobject.c b/Objects/methodobject.c
index b997746..7d70cc0 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -319,7 +319,7 @@
 }
 
 void
-PyCFunction_Fini(void)
+_PyCFunction_Fini(void)
 {
     (void)PyCFunction_ClearFreeList();
 }