GH10559: Minor improvement: Change read_excel sheet name
modify io/excel.py and relevant docs (io.rst) to use sheet_name for read_excel
but allow sheetname to still be used for backwards compatibility.  add test_excel
to verify that sheet_name and sheetname args produce the same result.

 - [X] closes #10559 
 - [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/16442?src=pr&el=h1) Report
> Merging [#16442](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/49ec31bbaeca81a6f58fc1be26fe80f3ac188cdd?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `80%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16442/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16442      +/-   ##
==========================================
- Coverage   90.42%   90.42%   -0.01%     
==========================================
  Files         161      161              
  Lines       51023    51025       +2     
==========================================
  Hits        46138    46138              
- Misses       4885     4887       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.26% <80%> (-0.01%)` | :arrow_down: |
| #single | `40.17% <40%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `62.22% <80%> (-0.04%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.05% <0%> (-0.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16442?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/16442?src=pr&el=footer). Last update [49ec31b...5a59cd0](https://codecov.io/gh/pandas-dev/pandas/pull/16442?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/16442?src=pr&el=h1) Report
> Merging [#16442](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/49ec31bbaeca81a6f58fc1be26fe80f3ac188cdd?src=pr&el=desc) will **decrease** coverage by `2.16%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16442/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16442      +/-   ##
==========================================
- Coverage   90.42%   88.26%   -2.17%     
==========================================
  Files         161      161              
  Lines       51023    51025       +2     
==========================================
- Hits        46138    45037    -1101     
- Misses       4885     5988    +1103
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.26% <100%> (ø)` | :arrow_up: |
| #single | `?` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `62.31% <100%> (+0.05%)` | :arrow_up: |
| [pandas/io/sql.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL3NxbC5weQ==) | `46.66% <0%> (-48.14%)` | :arrow_down: |
| [pandas/core/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vcHl0YWJsZXMucHk=) | `62.38% <0%> (-29.97%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `64.24% <0%> (-28.83%)` | :arrow_down: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `72.72% <0%> (-15.16%)` | :arrow_down: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `78.57% <0%> (-7.15%)` | :arrow_down: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `84.03% <0%> (-4.46%)` | :arrow_down: |
| [pandas/io/clipboard/clipboards.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2NsaXBib2FyZC9jbGlwYm9hcmRzLnB5) | `22.78% <0%> (-3.8%)` | :arrow_down: |
| [pandas/io/formats/console.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY29uc29sZS5weQ==) | `66.66% <0%> (-3.04%)` | :arrow_down: |
| [pandas/io/formats/printing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvcHJpbnRpbmcucHk=) | `87.61% <0%> (-1.77%)` | :arrow_down: |
| ... and [6 more](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16442?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/16442?src=pr&el=footer). Last update [49ec31b...4c6da13](https://codecov.io/gh/pandas-dev/pandas/pull/16442?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @abarber4gh thanks, nice job! If you want to check the docs http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.read_excel.html when [this build](https://travis-ci.org/pandas-dev/pandas/jobs/235365169) finishes, I'm curious to see how the `.. deprecated:` directive looks.