docs: remove references to removed setup_install_args

[branchv]

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Gemini 3.5 Flash provided some feedback on how best to organize the new historical context section

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Language::Python.setup_install_args was completely removed in 6.0 and replaced by std_pip_args. Our docs were lagging behind, so this updates them accordingly.

• Common-Issues.md: Removed the entire easy-install.pth section, which is no longer relevant as easy_install is long gone
• Homebrew-and-Python.md: Removed the setup.py install reference as this shouldn't be used anymore
• Python-for-Formula-Authors.md: Removed a lot of cruft that was added way back in 2014 to explain setup.py-related idiosyncrasies that are no longer relevant in today's pyproject.toml world. Instead, I just briefly summarized the transition from setup.py to pip but maybe that's not even useful here, not sure

[Copilot]

Pull request overview

This PR updates Homebrew’s Python documentation to remove references to the removed Language::Python.setup_install_args helper and modernize guidance around pip/pyproject.toml packaging.

Changes:

• Removes outdated setup.py/easy_install-era guidance and references to setup_install_args.
• Updates examples and wording to reflect current pip/std_pip_args usage.
• Replaces a large historical “rabbit hole” section with a shorter “Historical context” summary.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/Python-for-Formula-Authors.md	Removes setup_install_args discussion and rewrites packaging history/context.
docs/Homebrew-and-Python.md	Removes python3 setup.py install mention; keeps guidance focused on pip.
docs/Formula-Cookbook.md	Updates the Python resource install example to use pip/std_pip_args.
docs/Common-Issues.md	Removes obsolete easy-install.pth linking issue section and its outdated remediation.

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[p-linnane]

Great updates @branchv!