`pd.Index` should not support `float16`

[cmp0xff]

The following constructions are invalid but are not reported with the current pandas-stubs:

import pandas as pd
import numpy as np

# numpy float16
pd.Index([1.0], dtype=np.half)
pd.Index([1.0], dtype="half")
pd.Index([1.0], dtype="float16")
pd.Index([1.0], dtype="e")
pd.Index([1.0], dtype="f2")

We can start to fix them after #1493 has been merged.