clippy nightly warns on some function argument conversions

[alex]

Consider the following minimal method:

#[pyo3::prelude::pyclass]
struct Pool;

#[pyo3::pymethods]
impl Pool {
    fn acquire(slf: pyo3::Py<Self>) -> pyo3::Py<Self> {
        slf
    }
}

With the latest nightly, it produces the following clippy warning:

/t/t ❯❯❯ cargo +nightly clippy
warning: use of a fallible conversion when an infallible one could be used
 --> src/lib.rs:6:21
  |
6 |     fn acquire(slf: pyo3::Py<Self>) -> pyo3::Py<Self> {
  |                     ^^^^^^^^^^^^^^ help: use: `From::from`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_fallible_conversions
  = note: `#[warn(clippy::unnecessary_fallible_conversions)]` on by default

warning: `t` (lib) generated 1 warning

[davidhewitt]

Fixed in #3564