bpo-18578: Rename and document test.bytecode_helper as test.support.bytecode_helper (GH-15168)

Rename and document test.bytecode_helper as test.support.bytecode_helper
diff --git a/Lib/test/test_peepholer.py b/Lib/test/test_peepholer.py
index c90a532..47dee33 100644
--- a/Lib/test/test_peepholer.py
+++ b/Lib/test/test_peepholer.py
@@ -1,7 +1,7 @@
 import dis
 import unittest
 
-from test.bytecode_helper import BytecodeTestCase
+from test.support.bytecode_helper import BytecodeTestCase
 
 
 def count_instr_recursively(f, opname):