Set requires-python = "==3.13.*"

I had some difficulty getting the dependencies installed on my Intel-based MacBook Pro. Currently, `pyproject.toml` has `requires-python = ">=3.13"` so running `uv sync` creates a virtual environment using Python 3.14. The `uv.lock` file however only contains resolves for the `grpcio` dependency for Python 3.13:

https://github.com/developmentseed/geo-assistant/blob/e3373026d6626077bff67c047088d5813dae6447/uv.lock#L1203-L1216

As a result `uv` attempts to build `grpcio` wheels which fails on my Intel-based MacBook Pro with an error from the system `clang++` compiler.

I was able to resolve this by setting `requires-python = "==3.13.*"` which allows the available `grpcio` wheels to be installed.