Series[EA].astype always casts intermediate to NumPy array
Calling `pd.Series(EA).astype(object)` will always generate an intermediate NumPy array and never delegate to the `astype` method of the `ExtensionArray`.

Callstack is as follows:

1. `pd.Series(EA).astype()`
2. (omitting some intermediates)
3. `pandas.core.internals.Block._astype` calls `self.get_values()` in https://github.com/pandas-dev/pandas/blob/4274b840e64374a39a0285c2174968588753ec35/pandas/core/internals.py#L661
    (introducted by https://github.com/pandas-dev/pandas/pull/20581/files)
4. `pandas.core.internals.ExtensionBlock.get_values` then casts the ExtensionArray to a `numpy.array`: https://github.com/pandas-dev/pandas/blob/4274b840e64374a39a0285c2174968588753ec35/pandas/core/internals.py#L1937

Expected behaviour would have been that `astype` is called on the `ExtensionArray` which then can do the casting on its own. Currently I have the problem that my underlying storage (ExtensionArray backed by Arrow) is not numpy-compatible and thus everything turns into `np.array(…, dtype=object)` before it is casted.

Happy to fix this on my own but I would need a pointer on what the correct approach is, i.e. where one should delegate to `ExtensionArray.astype`.

#### Output of ``pd.show_versions()``

Ran into this with 0.23.0 but the code has not changed in master in this area.

<details>

```
INSTALLED VERSIONS                                                                                                                                    [6/1805]
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-112-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8
LOCALE: de_DE.UTF-8

pandas: 0.23.0
pytest: 3.6.0
pip: 9.0.3
setuptools: 39.2.0
Cython: None
numpy: 1.14.2
scipy: None
pyarrow: 0.9.0
xarray: None
IPython: 6.4.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```

</details>
