Suppress question_mark Clippy false positive

rust-lang/rust-clippy#7859

    error: this block may be rewritten with the `?` operator
       --> src/ser.rs:871:9
        |
    871 | /         if self.writer.write_all(s.as_bytes()).is_err() {
    872 | |             return Err(fmt::Error);
    873 | |         }
        | |_________^ help: replace it with: `self.writer.write_all(s.as_bytes())?;`
        |
        = note: `-D clippy::question-mark` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

Clippy's suggested replacement does not compile.

    error[E0277]: `?` couldn't convert the error to `std::fmt::Error`
       --> src/ser.rs:871:44
        |
    871 |         self.writer.write_all(s.as_bytes())?;
        |                                            ^ the trait `std::convert::From<std::io::Error>` is not implemented for `std::fmt::Error`
        |
        = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
        = note: required because of the requirements on the impl of `std::ops::FromResidual<std::result::Result<std::convert::Infallible, std::io::Error>>` for `std::result::Result<(), std::fmt::Error>`
    note: required by `std::ops::FromResidual::from_residual`
       --> nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/try_trait.rs:339:5
        |
    339 |     fn from_residual(residual: R) -> Self;
        |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/lib.rs

@@ -69,6 +69,8 @@
 #![deny(missing_docs)]
 // Suppressed clippy_pedantic lints
 #![allow(
+    // buggy
+    clippy::question_mark, // https://github.com/rust-lang/rust-clippy/issues/7859
     // private Deserializer::next
     clippy::should_implement_trait,
     // things are often more readable this way