support serializing nullable float data

[mattijn]

Fixes #2398.
This PR introduces support for serialising pandas nullable data types for float data (Float32Dtype and Float64Dtype) that were introduced in pandas 1.2.0.

import pandas as pd
import altair as alt

df = pd.DataFrame({'x': [1, 2, 3, 4, 5, 6, 7], 'y': [10, 30, None, 15, None, 40, 20]}, 
                  dtype=pd.Float32Dtype()) # pd.Float64Dtype()

alt.Chart(df).mark_line(point=True).encode(x='x', y='y')

[jakevdp]

Thanks - this looks fine. Once the black formatting check passes, we can get this merged.

[jakevdp]

Looks great, thanks!