Update syn requirement from 1 to 2

[dependabot[bot]]

Updates the requirements on syn to permit the latest version.

Release notes

Sourced from syn's releases.

2.0.29

• Partially work around rust-analyzer bug (rust-lang/rust-analyzer#9911)

Commits

• b387b13 Release 2.0.29
• 1faea07 Partially work around ridiculous rust-analyzer behavior
• 24d6ef4 Merge pull request #1494 from dtolnay/primitive
• 47dc5d0 Switch to using primitives in macro-generated code through std::primitive
• 020fe20 Update test suite to nightly-2023-08-05
• f561339 Update test suite to nightly-2023-08-04
• 44c5fdd Release 2.0.28
• 65aa662 Merge pull request #1491 from dtolnay/expectedexpr
• 91c84f6 Fix trivial difference between full and non-full expression parse errors
• f79ba15 Update test suite to nightly-2023-07-29
• Additional commits viewable in compare view

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