Out of curiosity, could you try the following patch?

Index: Lib/test/test_zlib.py
===================================================================
--- Lib/test/test_zlib.py	(révision 88500)
+++ Lib/test/test_zlib.py	(copie de travail)
@@ -70,7 +70,7 @@
         with open(support.TESTFN, "wb+") as f:
             f.seek(_4G)
             f.write(b"asdf")
-            f.flush()
+        with open(support.TESTFN, "rb") as f:
             self.mapping = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
 
     def tearDown(self):