DEPR: Deprecate parse_cols in read_excel
Will now use "usecols" just like in read_csv.
closes #4988.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=h1) Report
> Merging [#17774](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81694dce171ecd93a65e32ed455612ee967d3951?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/17774/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17774      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49872    49873       +1     
==========================================
- Hits        45514    45506       -8     
- Misses       4358     4367       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <100%> (ø)` | :arrow_up: |
| #single | `40.24% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.4% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17774?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/17774?src=pr&el=footer). Last update [81694dc...517b726](https://codecov.io/gh/pandas-dev/pandas/pull/17774?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/17774?src=pr&el=h1) Report
> Merging [#17774](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81694dce171ecd93a65e32ed455612ee967d3951?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/17774/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17774      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49872    49873       +1     
==========================================
- Hits        45514    45506       -8     
- Misses       4358     4367       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <100%> (ø)` | :arrow_up: |
| #single | `40.24% <27.27%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.4% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17774?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/17774?src=pr&el=footer). Last update [81694dc...62d6cac](https://codecov.io/gh/pandas-dev/pandas/pull/17774?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Besides deciding whether to leave #4988 open after merging, it's all green.  PTAL. looks fine. just run the excel tests locally and see if you are getting any deprecation warnings (and then suppress or better yet change). also on the docs you may need to change some older code to a code-block (and may need to fix text in the docs in any event). @jreback : I replaced all instances of `parse_cols` in the `io.rst` doc.  Also, I'm getting no warnings when running the tests.  GitHub <a href="https://github.com/pandas-dev/pandas/search?utf8=%E2%9C%93&q=parse_cols&type=">search</a> seems to indicate that that is all.

If all is good, can merge this in. thanks @gfyoung !