fixes #3561 -- silence new clippy warning

newsfragments/3564.fixed.md

@@ -0,0 +1 @@
+Silenced new `clippy::unnecessary_fallible_conversions` warning when using a `Py<Self>` `self` receiver

pyo3-macros-backend/src/method.rs

@@ -182,6 +182,7 @@ impl SelfType {
                             .map_err(::std::convert::Into::<_pyo3::PyErr>::into)
                             .and_then(
                                 #[allow(clippy::useless_conversion)]  // In case slf is PyCell<Self>
+                                #[allow(unknown_lints, clippy::unnecessary_fallible_conversions)]  // In case slf is Py<Self> (unknown_lints can be removed when MSRV is 1.75+)
                                 |cell| ::std::convert::TryFrom::try_from(cell).map_err(::std::convert::Into::into)
                             )