Add state code from FIPS

[MaxGhenis]

CPS and ACS provide state FIPS code. To tie that to the two-letter state code we use, we could use the us package as:

two_letter_code = pd.Series(fips).apply(
    lambda x: us.states.lookup(str(x).zfill(2)).name
).tolist()

per unitedstates/python-us#42.

[nikhilwoodruff]

Ah- just implemented (or rather, copilot wrote and I checked) for the ACS in #690