Edit: Yes it does, as ynik rightly points out, and documented here https://pyo3.rs/v0.19.2/types#gil-lifetimes-mutability-and-p... or here https://docs.rs/pyo3/0.19.2/pyo3/marker/struct.Python.html#o...
It's essentially a dummy parameter that serves as proof that the GIL is held. You need to provide it to PyO3 when calling back into the Python runtime, that way the compiler ensures the GIL is still held where required.