GH-92184: Convert os.altsep to '/' in filenames when creating ZipInfo objects (#92185)

This causes the zipfile module to also consider the character defined by
`os.altsep` (if there is one) to be a path separator and convert it to a
forward slash, as defined by the zip specification.

A logical no-op on all known platforms today as os.altsep is currently only set to a meaningful value on Windows (where it is "/").