Fix clippy::explicit_iter_loop (pedantic)

“it is more concise to loop over references to containers instead of
using explicit iteration methods”

https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/flake8_bugbear/plugins/loop_variable_overrides_iterator.rs

@@ -31,7 +31,7 @@ where
             }
             ExprKind::Lambda { args, body } => {
                 visitor::walk_expr(self, body);
-                for arg in args.args.iter() {
+                for arg in &args.args {
                     self.names.remove(arg.node.arg.as_str());
                 }
             }