gh-134158: Fix PyREPL coloring of double braces in f/t-strings

[loic-simon]

This PR add a special-case in _pyrepl.utils.Span.from_token to correctly color double braces in f-strings or t-strings.

Before:

After:

• Issue: PyREPL doesn't correctly color double braces in f-strings / t-strings #134158

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[StanFromIreland]

Looking at the after it still does not seem correct, the fifth and sixth example should both have a white pair of braces.

[loic-simon]

Looking at the after it still does not seem correct, the fifth and sixth example should both have a white pair of braces.

Why do you think so? They only contain literal braces, no interpolation delimiters.

We could add a specific color for double-braces, to convey the meaning they are "special", like eg. VSCode default theme does:

but it should be a different color from interpolation delimiters, and I'm not sure we have this level of detail in PyREPL (for example "\u1234" isn't colored differently that other strings literals, whereas it is in VSCode)

[python-cla-bot]

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[miss-islington-app]

Thanks @loic-simon for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-134227 is a backport of this pull request to the 3.14 branch.