gh-72680: Fix false positives when using zipfile.is_zipfile() (GH-134250)

bpo-28494: Improve zipfile.is_zipfile reliability

The zipfile.is_zipfile function would only search for the EndOfZipfile
section header. This failed to correctly identify non-zipfiles that
contained this header. Now the zipfile.is_zipfile function verifies
the first central directory entry.

Changes:
* Extended zipfile.is_zipfile to verify zipfile catalog
* Added tests to validate failure of binary non-zipfiles
* Reuse 'concat' handling for is_zipfile

Co-authored-by: John Jolly <john.jolly@gmail.com>