bpo-40275: Move requires_hashdigest() to test.support.hashlib_helper (GH-19716)

Add a new test.support.hashlib_helper submodule.
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 99196f6..25e9e93 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -11,7 +11,7 @@
 import tarfile
 
 from test import support
-from test.support import script_helper, requires_hashdigest
+from test.support import script_helper
 
 # Check for our compression modules.
 try: