Modify ticket in logo (typeddjango#1502)

@lolbat mentioned on Mastodon that it looks like a razor blade: https://fosstodon.org/@lolbat/110379380071734218 . Now I've seen it, I can't unsee it.

I think we should also avoid the razor blade appearance because it may trigger those who have some relationship with suicide.

I've modified the icon to use clipped-off corners, which is another common ticket design (see image search for "ticket").