Allow many_single_char_names still produces warnings

[bluss]

#![allow(clippy::many_single_char_names)] does not seem to suppress the lint, as it should,

It produces warnings like

warning: 6 bindings with single-character names in scope
   --> src\dimension\mod.rs:422:17
    |
422 | fn extended_gcd(a: isize, b: isize) -> (isize, (isize, isize)) {
    |                 ^         ^
...

Lints allowed in the code:

#![allow(
    clippy::many_single_char_names,
    clippy::deref_addrof,
    clippy::unreadable_literal,
    clippy::many_single_char_names
)]

Reproduced at ndarray commit 78bd1d84b9baf26c82c299e3bada43f72aa04614

Clippy version clippy 0.0.212 (6651c1b 2020-04-15)

CI log with failure (clippy at the end) https://travis-ci.org/github/rust-ndarray/ndarray/jobs/678756560

Previous similar issue #3198

[flip1995]

This will be fixed with #5518

[bluss]

nice!