Doesn't build with `indexmap` `2.1`

[Vollkornaffe]

So, I'm not 100% sure this is an issue on this crate.
(It might also be a violation of semantic versioning on indexmap side)

If you add the minor version to the dependency, this should be fixed.
E.g., 2.5.

You can reproduce the issue by pinning the version to 2.1, and you'll get something like

error[E0599]: the method `get_index_mut2` exists for struct `IndexMap<I, P, H>`, but its trait bounds were not satisfied
  --> src/double_priority_queue/iterators.rs:93:14 

[garro95]

Thanks for reporting.