Whatsnew cleanup
A few changes:

- Updated release.rst
- Update 0.21.0.txt
- Fixed CategoricalDtype API docs and references (this introduced more warnings into the doc build. I plan to spend a day or two with sphinx sometime in the next month fixing all these and getting us on the latest numpydoc and sphinx)
- Adds an `announce.py` script for generating the contributors list.
Hello @TomAugspurger! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on October 12, 2017 at 21:20 Hours UTC lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@92db5c9`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17858/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17858   +/-   ##
=========================================
  Coverage          ?   91.21%           
=========================================
  Files             ?      163           
  Lines             ?    50038           
  Branches          ?        0           
=========================================
  Hits              ?    45640           
  Misses            ?     4398           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <ø> (?)` | |
| #single | `40.26% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.14% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17858?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/17858?src=pr&el=footer). Last update [92db5c9...081c306](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=h1) Report
> Merging [#17858](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92db5c9c461ca4c649cfddcc6b0fb810cec18595?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17858/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17858      +/-   ##
==========================================
- Coverage   91.23%   91.21%   -0.02%     
==========================================
  Files         163      163              
  Lines       50069    50069              
==========================================
- Hits        45678    45669       -9     
- Misses       4391     4400       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <ø> (ø)` | :arrow_up: |
| #single | `40.26% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.14% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17858?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/17858?src=pr&el=footer). Last update [92db5c9...081c306](https://codecov.io/gh/pandas-dev/pandas/pull/17858?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
