BUG: Fix Series doesn't work in pd.astype(). Now treat Series as dict.
 - [x] closes #16717 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16725?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@794e060`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16725/graphs/tree.svg?height=150&token=eZ4WkYLtcO&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16725?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #16725   +/-   ##
=========================================
  Coverage          ?   90.93%           
=========================================
  Files             ?      161           
  Lines             ?    49260           
  Branches          ?        0           
=========================================
  Hits              ?    44797           
  Misses            ?     4463           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.71% <100%> (?)` | |
| #single | `40.18% <50%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16725?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16725?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.27% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16725?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/16725?src=pr&el=footer). Last update [794e060...fbd0755](https://codecov.io/gh/pandas-dev/pandas/pull/16725?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback Done as your suggestion. thanks!