handle UTF8 filenames w/o UTF8 flag?

[obfusk]

• it looks like zipflinger never sets the flag and just assumes UTF8.
• python needs the flag (or passing metadata_encoding in py >= 3.11) to decode properly (instead of using cp437).
• v1 signature file names are most likely ASCII and unaffected.
• copying is probably mostly unaffected:
  • as long as filename.decode("cp437").encode("cp437") == filename, which seems to be the case;
  • error messages containing file names might be affected though.