Include pitSmallData.csv in MANIFEST.in

[jdebacker]

In order for data files to be included with a Python package, they must be specified in the MANIFEST.in file. Currently, the pitSmallData.csv and the pit_weightsSD.csv files are not included in MANIFEST.in. They are therefore not installed as part of the taxcalc package when it is built from e.g., a pip install directly from GitHub. This has caused issues for several trying to run OG-India as seen in this discussion in the OG-India repo.

Related Stack Overflow issue for reference.

cc @sebastiansajie @rickecon