I just merged https://github.com/python/cpython/commit/93602e3af70d3b9f98ae2da654b16b3382b68d50 as the fix for issue #29537, so 3.5.4+ should handle legacy bytecode files without any complaints (and redistributors can backport it to 3.5.3 as needed).

For *this* issue, the main requested change to the proposed test case is to simplify it to use Serhiy's suggested approach: just define a single "EXPECTED_MAGIC_NUMBER" in the test suite, and have that be different on different maintenance branches.

For branches in alpha and beta phase, the test case should be skipped with a message like "Bytecode stability is not checked in alpha & beta releases"