feat(python): Support overriding from env vars for `single_version_platform_override`

[meteorcloudy]

• Similar to python.defaults's python_version_env attribute, we add python_version_env, sha256_env, strip_prefix_env, url_env for corresponding attributes of single_version_platform_override, allowing reading those values from env vars.
• Allow the python version specified to be a minor version, which will be mapped to a full version with MINOR_MAPPING from //python:versions.bzl.

This feature will make sure JAX's HERMETIC_PYTHON_* env vars keep working after migrating to Bzlmod. https://docs.jax.dev/en/latest/developer.html#custom-hermetic-python-examples

[gemini-code-assist[bot]]

Summary of Changes

Hello @meteorcloudy, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the flexibility of Python toolchain configuration within Bzlmod by introducing the ability to override critical attributes like Python version, SHA256 checksum, strip prefix, and download URLs directly from environment variables. This change is crucial for maintaining compatibility with existing workflows, such as JAX's hermetic Python setups, ensuring a smooth transition to Bzlmod while providing external control over toolchain definitions.

Highlights

• Environment Variable Overrides: Introduced new _env attributes (python_version_env, sha256_env, strip_prefix_env, url_env) for single_version_platform_override to allow values to be read from environment variables.
• JAX Compatibility: This feature specifically addresses the need for JAX's HERMETIC_PYTHON_* environment variables to continue working after migrating to Bzlmod.
• Flexible Python Versioning: Enhanced python_version handling to support minor versions when read from environment variables, mapping them to full versions if available.

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[rickeylev]

I'm pretty mixed on this. I'd like to talk in https://github.com/bazel-contrib/rules_python/issues/3293 more to try and figure out how to glue things together better.

As is, what this implies is all the APIs we have that accept some sort of version, url, etc, arg should have a _env and _target variations of them. (so the pip, python, and uv bzlmod extensions, which encompasses a very large api surface).

Having python_version_env for single_version_platform_override is also a bit weird -- the API name means it applies to a specific version. Getting the version from an environment variable means its for an arbitrary version.

[meteorcloudy]

Yeah, I agree this is more of an API change that specifically address JAX/XLA's needs, happy to wait for a more proper solution.