tests/comparators/test_sevenz.py: amend 7zip version test for older 7z

Sergei Trofimovichrequested to merge
trofi/diffoscope:7z-version-fix into master

Without the change 7z test fails with assertion errors:

FAILED tests/comparators/test_sevenz.py::test_metadata_diff - AssertionError

This happens because version guard did not work for my ancient 7z:

$ 7z | head -n2

7-Zip [64] 17.05 : Copyright (c) 1999-2021 Igor Pavlov : 2017-08-28

THe change attempts to skip [64] identifier. Tested on 7z 17.05 only.

Merge request reports