BUG/API: Raise when extension class passed to astype
Closes https://github.com/pandas-dev/pandas/issues/17780
cc @jorisvandenbossche @jreback @toobaz 

For the moment, I went with raising when an extension type *class* is passed instead of an instance. I'm happy to change the behavior here though if anyone disagrees. `CategoricalDtype` has an obvious "default" of `CategoricalDtype()`. `DatetimeTZDtype` doesn't really have a good default, so I decided to consistently raise an exception. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=h1) Report
> Merging [#17796](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/22515f58c178cdb5cd38c4e56f26dc91c7053550?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17796/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17796      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       49967    49971       +4     
==========================================
- Hits        45593    45588       -5     
- Misses       4374     4383       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <100%> (ø)` | :arrow_up: |
| #single | `40.24% <50%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.38% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=footer). Last update [22515f5...e868d54](https://codecov.io/gh/pandas-dev/pandas/pull/17796?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
