Preliminary support for Pixi projects using pyproject.toml

[jherland]

NOTE: We do not currently differentiate between Conda dependencies and PyPI dependencies, meaning that we assume that a Conda dependency named FOO will map one-to-one to a Python package named FOO. This is certainly not true for Conda dependencies that are not Python packages, and it probably isn't even true for all Conda dependencies that do indeed include Python packages.

Commits:

• parse_requirements_txt: Add debug log messages for invalid lines
• parse_pyproject_toml: Support Pixi dependencies
• parse_pyproject_toml: Support Pixi pypi-dependencies
• parse_pyproject_toml: Support Pixi "feature" dependencies
• parse_pyproject_toml: Support Pixi "feature" pypi-dependencies
• parse_pyproject_toml: Ignore Pixi's self-dependency
• Add more Pixi tests
• Talk about Pixi in our docs
• sample_projects: Add Pixi project using pyproject.toml