Update askama requirement from 0.14.0 to 0.15.0 #129

Job	Run time
semver	42s
Code Coverage	9s
Clippy	3m 51s
Build Wasm Examples (basic)	54s
Build Wasm Examples (callback-example)	56s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 40s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	2m 44s
Rustfmt	15s
Build Book	3m 43s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	6m 55s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	2m 52s
Build Examples (customization/density_mapbox_example)	28s
Build Examples (shapes)	39s
Build Examples (3d_charts)	33s
Build Examples (customization/multiple_plots_example)	29s
Build Examples (basic_charts)	32s
Build Examples (custom_controls)	43s
Build Examples (customization/consistent_static_format_export)	1m 25s
Build Examples (ndarray)	27s
Build Examples (images)	54s
Build Examples (statistical_charts)	33s
Build Examples (scientific_charts)	28s
Build Examples (kaleido)	1m 21s
Build Examples (maps)	5m 59s
Build Examples (subplots)	31s
Build Examples (themes)	35s
Build Examples (financial_charts)	31s
	42m 49s

Provide feedback