Use venv and `uv` throughout in lieu of system Python env + `pip`

[SamuelMarks]

Description

uv is inconsistently used in the codebase. In addition to being a faster complete rewrite in Rust, uv also has a dependency resolver that always works; whereas the usual pip one is sometimes flakey. And we don't test various Python and uv systems alongside each other anyway, so we might as well just use uv and later we can setup explicit tests to ensure pip works in addition to uv pip.

Tests

CI and manual tests

Checklist

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