style: mark sankey tolerance constant as private

Agent-Logs-Url: https://github.com/sdpython/teachpyx/sessions/68526f45-813e-4c5a-959d-84afb8a8544a

Co-authored-by: xadupre <22452781+xadupre@users.noreply.github.com>

Author: Copilot

teachpyx/faq/faq_python.py

@@ -4,7 +4,7 @@
 import os
 import re
 
-FLOW_BALANCE_TOLERANCE = 1e-10
+_FLOW_BALANCE_TOLERANCE = 1e-10
 
 
 def entier_grande_taille():
@@ -871,7 +871,7 @@ def graph_sankey(
     """
     if len(flows) < 2:
         raise ValueError("flows must contain at least two values.")
-    if abs(sum(flows)) > FLOW_BALANCE_TOLERANCE:
+    if abs(sum(flows)) > _FLOW_BALANCE_TOLERANCE:
         raise ValueError("The sum of all flows must be 0.")
     if labels is None:
         labels = [None] * len(flows)