Disable buggy iter_not_returning_iterator lint

rust-lang/rust-clippy#8285

    error: this method is named `iter` but its return type does not implement `Iterator`
       --> src/map.rs:238:5
        |
    238 |     pub fn iter(&self) -> Iter {
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: `-D clippy::iter-not-returning-iterator` implied by `-D clippy::pedantic`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_not_returning_iterator

    error: this method is named `iter_mut` but its return type does not implement `Iterator`
       --> src/map.rs:246:5
        |
    246 |     pub fn iter_mut(&mut self) -> IterMut {
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_not_returning_iterator

src/lib.rs

@@ -321,6 +321,8 @@
 )]
 // Ignored clippy_pedantic lints
 #![allow(
+    // buggy
+    clippy::iter_not_returning_iterator, // https://github.com/rust-lang/rust-clippy/issues/8285
     // Deserializer::from_str, into_iter
     clippy::should_implement_trait,
     // integer and float ser/de requires these sorts of casts