[python]: Import polars-core, not polars in dependencies

[kylebarron]

I thought this would make compile times faster (since it doesn't compile polars-time, polars-ops, polars-io, or polars-lazy), but it looks inconclusive below

rm -rf target/release && time maturin build --release

This branch:

4006.51s user 202.46s system 856% cpu 8:11.40 total

master:

3896.06s user 207.13s system 801% cpu 8:31.87 total