compat: PandasArray not accepted by many pandas functions
In #24990 there was a brief discussion about backing Blocks with PandasArray objects.  In an experimental local branch I'm trying this and finding a handful of rough edges that can be smoothed out.  I'll update this list as I find more

- [ ]  `view` method and `size` property
- [ ] `dtypes.cast.astype_nansafe` breaks on `PandasArray`
- [ ] `assert PandasDtype('object') == np.object` fails
- [ ] `is_dtype_equal(dtype, dtype)` returns `False` for `PandasDtype` objects