python@3.*: remove stale skip_clean

[branchvincent]

• python@3.8: remove stale skip_clean
• python@3.9: remove stale skip_clean
• python@3.10: remove stale skip_clean
• python@3.11: remove stale skip_clean

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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Related to Homebrew/brew#18002

These do nothing afaict. Only pip3 is still around but that's installed in post_install which happens after cleaning. These originated from 7871f86 way back in 2013 to perserve the writable bit so pip could upgrade itself pip install -U pip.

python@3.12 is handled in #180606