use python 3.7 for flax self-push tests
# What does this PR do?

Use python 3.7 for flax self-push tests. This PR also removes`kenlm` installation since it was  causing some issues and is actually not required for flax tests.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15865). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks for working on it! Is the `apt` installation necessary when you already specified the Python version you wanted in the yaml?<|||||>> Is the apt installation necessary when you already specified the Python version you wanted in the yaml?

Aah good catch! No, it's not. It is necessary for `kenlm` but it's not required anymore.