API: Make `Series.array` a read-only NumpyExtensionEA when applicable

[mroeschke]

Mutating Series.values has been disabled for 3.0 with CoW becoming the default behavior, but Series.array is still mutable.

In [1]: In [1]: import pandas as pd
   ...: 
   ...: In [2]: s = pd.Series([1, 2, pd.NA])
   ...: 
   ...: In [3]: a = s.array

In [2]: a[:2] = 3

In [3]: a
Out[3]: 
<NumpyExtensionArray>
[3, 3, <NA>]
Length: 3, dtype: object

In [4]: s
Out[4]: 
0       3
1       3
2    <NA>
dtype: object

There's a comment in pandas/core/internals/blocks.py in external_values to make backing ExtensionArrays read-only

[mroeschke]

cc @phofl

[mroeschke]

Locally, I'm seeing a failure for obj.interpolate(..., inplace=True) if we make the underlying array read-only. Should we punt on this until we make progress on PDEP-8?