Add small data files to manifest

[jdebacker]

This PR addresses Issue #158 by including the pit_SmallData.csv and pit_weightsSD.csv files in the MANIFEST.in file so that they will be included when the taxcalc package is built.

[codecov-io]

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[rickecon]

@sebastiansajie . If these small changes look good, will you please merge them? This should solve the problem in OG-India Issue #18. Thanks.