--- a/mercurial/util.py	Thu Jun 18 22:23:22 2020 +0200
+++ b/mercurial/util.py	Fri Jun 19 20:42:14 2020 +0530
@@ -204,6 +204,8 @@
             b" update your code.)"
         ) % version
         warnings.warn(pycompat.sysstr(msg), DeprecationWarning, stacklevel + 1)
+        # on python 3 with chg, we will need to explicitly flush the output
+        sys.stderr.flush()
 
 
 DIGESTS = {