diff -r ab5fd39cb402 -r 298d4400ea68 mercurial/revlogutils/constants.py
--- a/mercurial/revlogutils/constants.py	Tue Jun 01 09:18:27 2021 +0200
+++ b/mercurial/revlogutils/constants.py	Thu Jun 03 16:12:03 2021 +0200
@@ -29,7 +29,9 @@
 
 ### main revlog header
 
-INDEX_HEADER = struct.Struct(b">I")
+# We cannot rely on  Struct.format is inconsistent for python <=3.6 versus above
+INDEX_HEADER_FMT = b">I"
+INDEX_HEADER = struct.Struct(INDEX_HEADER_FMT)
 
 ## revlog version
 REVLOGV0 = 0