CLN: Remove sentinel_factory() in favor of object()
- [x] xref https://github.com/pandas-dev/pandas/pull/25069#discussion_r253042097
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=h1) Report
> Merging [#25074](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5c40cf21e23cf6e9137c7073e927f745f4dd6bcb?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25074/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25074      +/-   ##
==========================================
+ Coverage   92.36%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52404    52415      +11     
==========================================
+ Hits        48405    48419      +14     
+ Misses       3999     3996       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <100%> (ø)` | :arrow_up: |
| #single | `42.87% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `98.37% <ø> (-0.04%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `99.34% <100%> (-0.01%)` | :arrow_down: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <100%> (ø)` | :arrow_up: |
| [pandas/io/clipboard/windows.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NsaXBib2FyZC93aW5kb3dzLnB5) | `11.11% <0%> (-0.52%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.82% <0%> (ø)` | :arrow_up: |
| [pandas/compat/pickle\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `75.6% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.27% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.36% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.6% <0%> (+0.01%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.13% <0%> (+0.09%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25074?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/25074?src=pr&el=footer). Last update [5c40cf2...6cb976e](https://codecov.io/gh/pandas-dev/pandas/pull/25074?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/25074?src=pr&el=h1) Report
> Merging [#25074](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5c40cf21e23cf6e9137c7073e927f745f4dd6bcb?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25074/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25074      +/-   ##
==========================================
- Coverage   92.37%   92.36%   -0.01%     
==========================================
  Files         166      166              
  Lines       52403    52399       -4     
==========================================
- Hits        48405    48400       -5     
- Misses       3998     3999       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.79% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.88% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `98.37% <ø> (-0.04%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `99.34% <100%> (ø)` | :arrow_up: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25074/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25074?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/25074?src=pr&el=footer). Last update [5c40cf2...139cc84](https://codecov.io/gh/pandas-dev/pandas/pull/25074?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 linting issue, otherwise lgtm. ping on green. @jreback : green thanks @jschendel 