Homebrew-and-Python: more PEP 668 guidance

[woodruffw]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This adds an additional warning to our Python documentation, emphasizing that PEP 668 is in effect for 3.12+ and that the guidance for python -m pip install ... doesn't work on 3.12+.

Closes Homebrew/homebrew-core#176450.