ENH: Use 'Y' as an alias for end of year
Redo of #16958.
Closes #9313.

cc @imih 

Additional components:

- [x] `whatsnew` mention
- [x] doc in `timeseries.rst`
- [x] tests for 'YS' and `date_range`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=h1) Report
> Merging [#16978](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7ffe7fc21f3dc4ca444de9c83dbf61313b6986e2?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/16978/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16978      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49290    49292       +2     
==========================================
- Hits        44851    44844       -7     
- Misses       4439     4448       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <100%> (ø)` | :arrow_up: |
| #single | `40.19% <100%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96.63% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.76% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16978?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/16978?src=pr&el=footer). Last update [7ffe7fc...0ed47f6](https://codecov.io/gh/pandas-dev/pandas/pull/16978?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/16978?src=pr&el=h1) Report
> Merging [#16978](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b9a57fc99fcd63c55b041ea7c76f5c390c12aa0?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/16978/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16978      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49291    49298       +7     
==========================================
- Hits        44851    44850       -1     
- Misses       4440     4448       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (ø)` | :arrow_up: |
| #single | `40.19% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96.66% <100%> (+0.04%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.76% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16978?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/16978?src=pr&el=footer). Last update [7b9a57f...e6ec2b8](https://codecov.io/gh/pandas-dev/pandas/pull/16978?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 i would have simply taken the original commit and added things in another commit; no need to rebase the original away > i would have simply taken the original commit and added things in another commit; no need to rebase the original away

@jreback : That's fair.  Done. @jreback : PTAL @jreback : Everything is green.  PTAL lgtm http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#offset-aliases

needs updates for biz end freq Oops!  Done.