chore: name sankey flow balance tolerance constant

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

Author: Copilot

teachpyx/faq/faq_python.py

@@ -4,6 +4,8 @@
 import os
 import re
 
+FLOW_BALANCE_TOLERANCE = 1e-10
+
 
 def entier_grande_taille():
     """
@@ -869,7 +871,7 @@ def graph_sankey(
     """
     if len(flows) < 2:
         raise ValueError("flows must contain at least two values.")
-    if abs(sum(flows)) > 1e-10:
+    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)