feat: enable pyproject.toml as single source of truth for Python version

[janwinkler1]

• Enables using pyproject.toml as single source of truth for Python version via
  python.defaults(pyproject_toml=...) and pip.default(pyproject_toml=...).

• Creates python_version_from_pyproject repository for BUILD file usage:

load("@python_version_from_pyproject//:version.bzl", "PYTHON_VERSION")
compile_pip_requirements(python_version = PYTHON_VERSION, ...)

• Makes config_settings optional in pip.default(). Previously, users had
  to pass an empty list (config_settings = []) even when not using platform-specific
  configurations. Now it can be omitted entirely when not needed.

Changes:

• Added pyproject_toml attribute to python.defaults() and pip.default()
• Reads Python version from requires-python field (must be ==X.Y.Z format)
• Made config_settings optional in pip.default() (was mandatory, even for empty lists)
• Made python_version optional in pip.parse() when using pip.default(pyproject_toml=...)
• Extracted version parsing to shared Python script for maintainability

Requirements:

• requires-python = "==X.Y.Z" format in pyproject.toml
• Python >=3.11 or tomli package on host system

[gemini-code-assist]

Summary of Changes

Hello @janwinkler1, 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 Python ecosystem's integration by introducing pyproject.toml as a primary source for defining Python versions within Bazel projects. It streamlines configuration by allowing python.defaults() and pip.default() to automatically infer the Python version from pyproject.toml, reducing redundancy and potential for discrepancies. Additionally, it improves the developer experience by making config_settings optional in pip.default() and providing a dedicated repository for accessing the pyproject.toml-defined Python version in BUILD files.

Highlights

• Centralized Python Version Management: Introduced the pyproject_toml attribute to python.defaults() and pip.default(), enabling the Python version to be read directly from pyproject.toml's requires-python field. This establishes pyproject.toml as a single source of truth for Python versions across the project.
• BUILD File Integration for Python Version: A new repository rule, python_version_from_pyproject, has been created. This rule generates a version.bzl file that exports the PYTHON_VERSION constant, allowing BUILD files to import and use the Python version defined in pyproject.toml without hardcoding it.
• Simplified pip.default() Configuration: The config_settings attribute in pip.default() is now optional. Previously, users had to pass an empty list (config_settings = []) even when no platform-specific configurations were needed, but now it can be omitted entirely.
• Flexible pip.parse() Usage: The python_version attribute in pip.parse() is now optional when pip.default(pyproject_toml=...) is used, as the version can be automatically inferred from the pyproject.toml file specified in the defaults.
• Dedicated Version Extraction Logic: The logic for parsing and extracting the Python version from pyproject.toml has been moved into a shared Python script (pyproject_version_extractor.py), improving maintainability and reusability of the version parsing functionality.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and valuable feature by enabling pyproject.toml as a single source of truth for Python versions in both python.defaults() and pip.default(). This enhancement greatly improves consistency and maintainability across projects. The implementation includes a new repository rule (pyproject_version_repo) and utility functions to extract and validate the Python version from pyproject.toml. Additionally, config_settings in pip.default() has been made optional, which is a good usability improvement. The changes are well-tested with dedicated integration and priority tests, ensuring the new functionality works as expected and respects the defined priority order for Python version resolution. The code is clean, well-documented, and adheres to best practices.