feat: allow registering arbitrary settings for py_binary transitions

[rickeylev]

This implements the ability for users to add additional settings that py_binary, py_test,
and py_wheel can transition on.

There were three main use cases motivating this feature:

1. Making it easier to have multiple pypi dependency closures and shared dependencies.
2. Making it easier to override flags for py_wheel.
3. Making it easier to have per-target setting of things like bootstrap_impl, venv
   site packages, etc.

It also adds most of our config settings to the the transition inputs/outputs for those
rules, which allows users to per-target force particular settings without having to
use e.g. with_cfg to wrap a target with the desired transition settings. It also
lets use avoid adding dozens of attributes (one per setting); today there are
about 17 flags.

Under the hood, this works by having a bzlmod api that users can pass labels to. These
labels are put into a generated bzl file, which the rules load and add to their
list of transition inputs/outputs. On the target level, the config_settings attribute,
which is a dict[label, str], can be set to change the particular flags of interest.

Along the way...

• Create a common_labels.bzl file for the shared label strings
• Remove the defunct py_reconfig code in sh_py_run_test.

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! 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 configuration capabilities of rules_python by allowing py_binary, py_test, and py_wheel rules to apply arbitrary build settings on a per-target basis. This feature addresses common use cases such as managing multiple PyPI dependency closures, overriding flags for py_wheel builds, and setting specific configurations like bootstrap_impl or venv site packages for individual targets. The changes involve a new module extension for registering custom settings, a new attribute on the rules themselves, and internal refactoring to support this flexible configuration mechanism.

Highlights

• Arbitrary Setting Transitions: Introduces the ability for py_binary, py_test, and py_wheel rules to transition on arbitrary build settings, enabling more flexible configuration per target.
• New config_settings Attribute: Adds a config_settings attribute to executable Python rules, allowing users to specify custom flag values for individual targets without needing to wrap them in with_cfg.
• Module Extension for Custom Settings: Implements a new rules_python_config module extension with an add_transition_setting tag, providing a bzlmod API for users to register their own custom settings for transitions.
• Enhanced Default Transitions: Most existing rules_python configuration settings are now automatically included in the default transition inputs and outputs for py_binary, py_test, and py_wheel.
• Multi-PyPI Dependency Management Guide: Adds new documentation demonstrating how to manage common dependencies across multiple PyPI versions using the new config_settings feature and custom flags.
• Code Refactoring and Cleanup: Refactors and removes defunct py_reconfig code from sh_py_run_test.bzl, consolidating it into a dedicated py_reconfig.bzl file for better organization.
• Standardized Label Handling: Introduces new utility functions and files (common_labels.bzl, transition_labels.bzl) to standardize and normalize the handling of build setting labels used in transitions.

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[gemini-code-assist]

Code Review

This pull request introduces a powerful feature allowing users to register arbitrary settings for py_binary, py_test, and py_wheel to transition on. This is a significant improvement for managing complex dependency graphs, especially in monorepos with multiple PyPI dependency sets. The implementation is well-structured, introducing a new bzlmod extension, a repository rule to generate configuration, and refactoring existing rules to use this new mechanism. The addition of tests and documentation is also commendable. My feedback is mostly focused on minor corrections in the documentation to ensure clarity for users.

[rickeylev]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a powerful feature allowing users to register arbitrary settings for py_binary, py_test, and py_wheel transitions. The implementation is well-structured, introducing a new bzlmod extension //python/extensions:config.bzl and a config_settings attribute on the relevant rules. The changes are accompanied by thorough tests and new documentation, which is excellent.

My review focuses on a few minor documentation and docstring improvements to enhance clarity and correctness. Overall, this is a solid contribution that addresses several important use cases for rules_python users.

[aignas]

LGTM in general.