New testing setup

[steverab]

This PR adds the new testing setup. Concretely, it adds automatically running

• style checks for all code files
• the PyTorch tests in cleverhans/future/torch/tests
• the tutorials for PyTorch, TF2, and JAX

It also adds

• requirements specifications in requirements
• pre-commit hooks ensuring that a consistent code style is enforced for all contributors.

[alkaet]

This is going to be a large commit to review. @jonasguan @b-reckoner Let's split up the testing for this one?

[jonasguan]

Sounds good @alkaet , I can start by spinning up a fresh Azure machine and follow the CONTRIBUTING.md guide to check everything works as expected for contributors. I likely won't have time until next week though.

[jonasguan]

Glorious