Ignore return_self_not_must_use pedantic clippy lint

    warning: missing `#[must_use]` attribute on a method returning `Self`
       --> serde_derive_internals/src/attr.rs:204:5
        |
    204 | /     pub fn or(self, other_rules: Self) -> Self {
    205 | |         Self {
    206 | |             serialize: self.serialize.or(other_rules.serialize),
    207 | |             deserialize: self.deserialize.or(other_rules.deserialize),
    208 | |         }
    209 | |     }
        | |_____^
        |
        = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use
        = note: `-W clippy::return-self-not-must-use` implied by `-W clippy::pedantic`

    warning: missing `#[must_use]` attribute on a method returning `Self`
       --> serde_derive_internals/src/case.rs:112:5
        |
    112 | /     pub fn or(self, rule_b: Self) -> Self {
    113 | |         match self {
    114 | |             None => rule_b,
    115 | |             _ => self,
    116 | |         }
    117 | |     }
        | |_____^
        |
        = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use

serde_derive_internals/lib.rs

@@ -29,6 +29,7 @@
     clippy::missing_errors_doc,
     clippy::module_name_repetitions,
     clippy::must_use_candidate,
+    clippy::return_self_not_must_use,
     clippy::similar_names,
     clippy::single_match_else,
     clippy::struct_excessive_bools,