Update askama requirement from 0.15.0 to 0.16.0 #163

Job	Run time
Rustfmt	12s
Clippy	4m 36s
Code Coverage	20s
semver	54s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 23s
Build Book	3m 47s
Build Wasm Examples (basic)	1m 0s
Build Wasm Examples (callback-example)	1m 2s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 22s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	8m 35s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	4m 10s
Build Examples (3d_charts)	33s
Build Examples (customization/consistent_static_format_export)	1m 47s
Build Examples (images)	58s
Build Examples (basic_charts)	33s
Build Examples (kaleido)	1m 16s
Build Examples (customization/density_mapbox_example)	35s
Build Examples (custom_controls)	38s
Build Examples (shapes)	32s
Build Examples (customization/multiple_plots_example)	33s
Build Examples (maps)	45s
Build Examples (financial_charts)	39s
Build Examples (scientific_charts)	34s
Build Examples (subplots)	32s
Build Examples (statistical_charts)	36s
Build Examples (themes)	34s
Build Examples (ndarray)	36s
	43m 2s

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