fix: unittest discover path in Makefile and use uv for python

[Mahhheshh]

I was playing around, and discovered that running make test-py-core in root directory fails since there is no core directory, This fixes that.

What this doesn't fix is now unittest discover works and tests in py/src/braintrust/wrappers fail since they require optional dependencies.

And, py/Makefile does the same while installing the python deps, So I decided to do the same here. :P