It’s a chemistry pun about the process of oxidation, oxides (such as rust) have a chemical formula of XO3 where X is the oxidized element. The library is called Py Oxide (PyO3) where the oxidized element is Python
FeO3 (Iron Oxide) can be referred to as Rust and presumably the name PyO3 makes reference to that.
It’s a double pun because -O3 is the highest optimisation level for many compilers.