pyyaml: deprecate, disable in 3 months

[branchvincent]

• 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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For #167905

Remaining usage is in 2 disabled formulae

install: 1,620 (30 days), 5,346 (90 days), 222,583 (365 days)
install-on-request: 871 (30 days), 2,441 (90 days), 7,799 (365 days)