gradio: deprecate formula

[Amar1729]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• n/a Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• n/a Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• n/a Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Looks like gradio has been deprecated upstream: https://github.com/haecker-felix/Gradio

Owner/Readme points towards new project Shortwave (which actually should maybe be a cask rather than a formula anyway).

This is also one of the leftover formula depending on python (#47274).

[zachauten]

looks like gradio's only had a couple installs in the last month https://formulae.brew.sh/formula/gradio#default

[iMichka]

Hi. We do not delete the files right away now. Please use the disable DSL; see for example #54471.

The file will be removed later, probably after a few months.

[SMillerDev]

Can we mark it deprecated with the date first?

[iMichka]

Yes, even better; as it somewhat still works. The deprecation date is 16 Nov 2019.

[iMichka]

Thanks @Amar1729 !