bpo-40927: Fix test_binhex when run twice (GH-20764)

test_binhex now uses import_fresh_module() to ensure that it raises
DeprecationWarning each time.
diff --git a/Lib/test/test_binhex.py b/Lib/test/test_binhex.py
index 86ca37c..8595532 100644
--- a/Lib/test/test_binhex.py
+++ b/Lib/test/test_binhex.py
@@ -7,7 +7,7 @@
 from test import support
 
 with support.check_warnings(('', DeprecationWarning)):
-    import binhex
+    binhex = support.import_fresh_module('binhex')
 
 
 class BinHexTestCase(unittest.TestCase):