Improve import cubed time

[tomwhite]

The changes here take import cubed in a fresh venv from ~5s to ~3s.

time PYTHONPROFILEIMPORTTIME=1 uv run --reinstall import-cubed.py 2> import.log
uvx tuna import.log 

Contents of import-cubed.py:

# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "cubed @ /Users/tom/workspace/cubed",
# ]
# ///
import cubed