Add typings#976

Draft

cesco-fran wants to merge 7 commits intoopenfisca:masterfrom

cesco-fran:add-typings

Commits on Feb 16, 2021

Add typings to the variables module
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Add typings to the periods module
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Add typings to the holders module
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Add typings to the simulations module
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Add typings to the simulation_builder module
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Add typings to the populations module
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Add typings to the taxbenefitsystems module
cescofran
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