[MAINTAIN] Use uv for managing dependencies.

[RuanJohn]

Please describe what needs to be maintained?

Use uv for dependency management and use it to produce a locked requirements file with which Mava is guaranteed to run.

Why?

uv can produce great compiled and locked requirements files. It is also a lot faster than using pip and is a drop in replacement for it.

How?

Make a locked requirements file and updated the README to suggest uv. The locked requirements file will still work with normal pip, but uv will just be a lot faster.