No longer builds on last stable Rust

Since the release of Rust 1.29, debcargo no longer seems to build because there are two implementions of flatten() in scope:

   Compiling debcargo v2.2.7 (file:///.../debcargo)                                                                                                         
error[E0034]: multiple applicable items in scope
   --> src/config.rs:170:11
    |
170 |         ).flatten().map(|s| s.as_str())
    |           ^^^^^^^ multiple `flatten` found
    |
    = note: candidate #1 is defined in an impl of the trait `std::iter::Iterator` for the type `std::iter::Map<_, _>`
    = note: candidate #2 is defined in an impl of the trait `itertools::Itertools` for the type `_`