Bump mtl to 2.3.1
A step towards #21587 (closed): transformers-0.6 imply mtl-2.3, but not vice versa, so we can do upgrade in two steps: first bump mtl to 2.3.1, then bump transformers to 0.6.0.4. The latter is tracked in !9370 (closed).
A step towards #21587 (closed): transformers-0.6 imply mtl-2.3, but not vice versa, so we can do upgrade in two steps: first bump mtl to 2.3.1, then bump transformers to 0.6.0.4. The latter is tracked in !9370 (closed).