One thing you can do is, if you know someone who does have a Mac, have them do the chmod +x to the binary, zip it back up (right click > Compress Pumpkins.app will work) and send it back to you to upload to itch.io. The zip file created this way will preserve the executable permission. Users will still need to right click > Open the app to get past the "we can't check it for viruses" thing, but they at least won't need to use the Terminal on it themselves.
Edit: Not sure if this is expected, but Pumpkins.app also has no icon. Not an issue for running the app or anything, just wanted to point it out in case it should have one.