Update syn requirement from 2 to 3 #253

Job	Run time
Rustfmt	14s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	2m 3s
Clippy	2m 48s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	2m 3s
Build Wasm Examples (basic)	54s
semver	44s
Build Wasm Examples (callback-example)	57s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	2m 54s
Build Book	1m 20s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	4m 15s
Build Examples (customization/consistent_static_format_export)	1m 26s
Build Examples (customization/density_mapbox_example)	24s
Code Coverage	21s
Build Examples (custom_controls)	25s
Build Examples (customization/multiple_plots_example)	27s
Build Examples (basic_charts)	30s
Build Examples (kaleido)	1m 9s
Build Examples (3d_charts)	28s
Build Examples (financial_charts)	26s
Build Examples (themes)	24s
Build Examples (subplots)	22s
Build Examples (images)	48s
Build Examples (shapes)	27s
Build Examples (scientific_charts)	25s
Build Examples (ndarray)	26s
Build Examples (maps)	41s
Build Examples (statistical_charts)	28s
	27m 49s

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