Summary:
Hello again! This is the second [half](#82) of what we require in order to start using DotSlash.

This moves over the [code](https://github.com/ofek/dotslash-python) for the `dotslash` Python [package](https://pypi.org/project/dotslash/) I created over the weekend. The ownership transfer on PyPI is in progress.

The package serves a similar purpose to the existing `fb-dotslash` Node.js [package](https://www.npmjs.com/package/fb-dotslash) stored in the top level [`node`](https://github.com/facebook/dotslash/tree/main/node) directory. Some notes:

- Python 3.9 is EOL at the end of the month so I chose to require 3.10+.
- The package exposes a single `locate` function for finding the path to the binary that was installed by the installation. The logic was adapted from equivalents like UV's [finder](https://github.com/astral-sh/uv/blob/141369ce73b7b0b4e005b0f45107d13c828a99e0/python/uv/_find_uv.py). I optimized the path resolution to perform as little work as necessary and to do so lazily.
    ```
    ❯ docker run --rm -it python:3.14 bash
    root@8a442038a1c8:/# curl -fsSL https://github.com/sharkdp/hyperfine/releases/download/v1.19.0/hyperfine-v1.19.0-x86_64-unknown-linux-musl.tar.gz \
    | tar -xzO 'hyperfine-v1.19.0-x86_64-unknown-linux-musl/hyperfine' \
    | install -m 0755 /dev/stdin /usr/local/bin/hyperfine
    root@8a442038a1c8:/# pip install -qqq dotslash uv
    root@8a442038a1c8:/# hyperfine -m 100 --warmup 10 "python -m timeit -n 1 -r 1 -s 'from uv import find_uv_bin' 'find_uv_bin()'"
    Benchmark 1: python -m timeit -n 1 -r 1 -s 'from uv import find_uv_bin' 'find_uv_bin()'
      Time (mean ± σ):      33.6 ms ±   2.4 ms    [User: 25.3 ms, System: 7.6 ms]
      Range (min … max):    30.3 ms …  46.1 ms    100 runs

    root@8a442038a1c8:/# hyperfine -m 100 --warmup 10 "python -m timeit -n 1 -r 1 -s 'from dotslash import locate' 'locate()'"
    Benchmark 1: python -m timeit -n 1 -r 1 -s 'from dotslash import locate' 'locate()'
      Time (mean ± σ):      23.5 ms ±   1.0 ms    [User: 17.8 ms, System: 5.7 ms]
      Range (min … max):    22.1 ms …  26.9 ms    127 runs

    root@8a442038a1c8:/# # Show time it takes to locate binaries
    root@8a442038a1c8:/# python -m timeit -n 1 -r 1 -s "from uv import find_uv_bin" "find_uv_bin()"
    1 loop, best of 1: 7.71 msec per loop
    root@8a442038a1c8:/# # This includes the time of all imports (still faster)
    root@8a442038a1c8:/# python -m timeit -n 1 -r 1 -s "from dotslash import locate" "locate()"
    1 loop, best of 1: 2.86 msec per loop
    root@8a442038a1c8:/# # This excludes import times to match eager behavior of UV
    root@8a442038a1c8:/# python -m timeit -n 1 -r 1 -s "from dotslash import locate,_locate" "locate()"
    1 loop, best of 1: 1.14 msec per loop
    ```
- The shipped binary can be invoked directly with `python -m dotslash`. If there ever is a desire to represent paths as anything other than strings in the implementation, please [don't](astral-sh/ruff#9315).
- Builds require setting the `DOTSLASH_VERSION` environment variable to the desired release tag of DotSlash (the `v` is optional). This will be used as the package version and, by default, for fetching the appropriate release artifact.
- Rather than making HTTP requests during builds, you can set the `DOTSLASH_SOURCE` environment variable to the path to a directory containing the release artifacts. I strongly recommend that the release process uses that eventually (and the Node.js package's [build script](https://github.com/facebook/dotslash/blob/646102b468bb5e820e45d3d06a7ee63e13b51d66/node/scripts/build-package.js) gains support).
- I remove build instructions from the content of the PyPI project's landing page since the information wouldn't be useful for users.
- I moved the release jobs of the downstream packages into a separate workflow that only executes when called by the primary release workflow. I already configured Trusted Publishing on PyPI to use the `release-downstream.yml` workflow from this repository. You might find hazy details about their support for reusable workflows but [this comment](pypi/warehouse#11096 (comment)) provides the full picture. Basically, the claim they chose happens to work for the most nested called workflow when using reusable workflows but not the top level one. Whenever support for that happens there will be no user impact for existing supported scenarios.

 ---

cc sdwilsh bigfootjon for review as requested

Also, it's worth noting that PyPI was blocking the name `dotslash` due to certain [rules](https://github.com/pypi/warehouse/blob/main/warehouse/migrations/versions/d18d443f89f0_ultranormalize_name_function.py) and di very graciously provided the override 🙇‍♂️

Pull Request resolved: #87

Test Plan: CI

Reviewed By: bigfootjon

Differential Revision: D84891338

Pulled By: sdwilsh

fbshipit-source-id: 493086f3a0106d0c0e83c88eeb82bfe9ba277507