Rebased #5053 and fixed the impl to pass tests. Original PR and description below by @jjolly

Fix zipfile validation issue by ... providing more validation!

Originally, zipfile.is_zipfile() only checked the End Central Directory
signature. If the signature could be found in the last 64k of the file,
success! This produced false positives on any file with 'PK\x05\x06' in the
last 64k of the file - including PDFs and PNGs.

This is now corrected by actually validating the Central Directory location
and size based on the information provided by the End Central Directory
along with verifying the Central Directory signature of the first entry.

This should be sufficient for the vast number of zipfiles, but more could be
done to absolutely validate the zipfile content - such as validating all
local file headers and Central Directory entries.

• Issue: is_zipfile false positives #72680