DEPR: Deprecate usecols as int in read_excel
Follow-up to https://github.com/pandas-dev/pandas/pull/23544#discussion_r232441155

Pretty standard, cut and dry deprecation.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/io/excel.py`](https://github.com/gfyoung/pandas/blob/b8e98b6e4c9c468373ed5b8ade4e6fa1a3ad9ffd/pandas/io/excel.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_excel.py`](https://github.com/gfyoung/pandas/blob/b8e98b6e4c9c468373ed5b8ade4e6fa1a3ad9ffd/pandas/tests/io/test_excel.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23635?src=pr&el=h1) Report
> Merging [#23635](https://codecov.io/gh/pandas-dev/pandas/pull/23635?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a092e91f3c6d77c24518dff8fd829a96d26e5007?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23635/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23635?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23635   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51324    51324           
=======================================
  Hits        47339    47339           
  Misses       3985     3985
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.29% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23635?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/23635?src=pr&el=footer). Last update [a092e91...b8e98b6](https://codecov.io/gh/pandas-dev/pandas/pull/23635?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung can you add to list in #6581 