DEPR: Index.is_mixed

[jbrockmendel]

We only use it in one place, and removing it there doesn't break anything. Moreover, it behaves surprisingly:

>>> pd.Index(['a', np.nan, 'b']).is_mixed()
True

>>> Index([0, 'a', 1, 'b', 2, 'c']).is_mixed()
False

[jorisvandenbossche]

Fine with me to deprecate. People can always use pd.api.types.infer_dtype directly.