If you're trying to figure out the origin of a Rust project's name, the safest bet is always to choose the one that's a reference to metal.
i am original author of pyo3. Yuri Selivanov (author of uvloop and edgedb) suggested pyo3 name.
Oh, I know, I wasn't trying to correct you or anything. I was just adding on to the correct answer to point out that PyO3's naming scheme is part of a popular trend in Rust libraries.