Constrain tract to <0.21.7 so libDF compiles

[svaningelgem]

cargo build --features tract does not compile on a fresh checkout today.

libDF/Cargo.toml asks for tract-core = "^0.21.4" and uses ndarray ^0.15. But tract-data switched from ndarray ^0.15.3 to ^0.16 in 0.21.7, so cargo resolves to 0.21.17 and the two Axis types stop being the same type:

error[E0308]: mismatched types
   --> libDF/src/tract.rs:443:30
    |
443 |         let ch = spec.len_of(Axis(0));
    |                       ------ ^^^^^^^ expected `tract_core::ndarray::Axis`,
    |                                      found `ndarray::Axis`

Same at tract.rs:446 and tract.rs:535.

Where the boundary is, from the published metadata:

tract-data	ndarray
0.21.4	^0.15.3
0.21.5	^0.15.3
0.21.7	^0.16
0.21.9 … 0.21.17	^0.16

This PR is the minimal fix — hold tract on the ndarray-0.15 side so the build works again. Verified on Windows: cargo build --features tract --no-default-features now compiles, picking tract-core 0.21.4.

Bumping libDF to ndarray 0.16 instead would be the more forward-looking change, and would let tract move on rather than pinning it back. It touches transforms.rs and dataset.rs too, though, so it seemed better to leave that call to you than to bundle it in here. Happy to do that version instead if you prefer.

Found while exporting DeepFilterNet3 to ONNX and running it through your tract runner — which, once it compiles, works very well. Thanks for the model.