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

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.

tests/comparators/test_sevenz.py

@@ -38,7 +38,12 @@ def sevenz_version():
         out = subprocess.check_output(["7z"])
     except subprocess.CalledProcessError as e:
         out = e.output
-    return out.decode("UTF-8").split()[1].strip()
+    words = out.decode("UTF-8").split()
+    # 7zip 17.04 returns version after "[64]" identifier:
+    #   "7-Zip [64] 17.05 : Copyright (c) 1999-2021 Igor Pavlov : 2017-08-28"
+    if words[1].startswith("["):
+        return words[2].strip()
+    return words[1].strip()
 
 
 def test_identification(sevenza):