API: Default ExtensionArray.astype
I need this to progress cleanly on https://github.com/pandas-dev/pandas/pull/19558 and (which is blocking https://github.com/pandas-dev/pandas/pull/19520).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=h1) Report
> Merging [#19604](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b83512773ab0dd3908cf2ef5ccaddcd8e0337c64?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19604/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19604      +/-   ##
==========================================
- Coverage   91.62%    91.6%   -0.03%     
==========================================
  Files         150      150              
  Lines       48790    48798       +8     
==========================================
- Hits        44703    44699       -4     
- Misses       4087     4099      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.73% <66.66%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `60.6% <100%> (+3.93%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19604/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19604/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.42% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/19604/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `96.25% <0%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=footer). Last update [b835127...d136227](https://codecov.io/gh/pandas-dev/pandas/pull/19604?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 cc @jorisvandenbossche if you have thoughts.

This now just does `np.array`. It'll work if the underlying items can be converted to `dtype`.