WIP: add mode(dropna=True)
- [x] closes #17534
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @fulldecent! Thanks for submitting the PR.

 - In the file [`pandas/core/algorithms.py`](https://github.com/pandas-dev/pandas/blob/fb4204132641ae29f24c6084a9186bb5b397971e/pandas/core/algorithms.py), following are the PEP8 issues :

> [Line 671:1](https://github.com/pandas-dev/pandas/blob/fb4204132641ae29f24c6084a9186bb5b397971e/pandas/core/algorithms.py#L671): [E266](https://duckduckgo.com/?q=pep8%20E266) too many leading '#' for block comment





 can you rebase and update # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@c65a0f5`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17547/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17547   +/-   ##
=========================================
  Coverage          ?    91.2%           
=========================================
  Files             ?      163           
  Lines             ?    49606           
  Branches          ?        0           
=========================================
  Hits              ?    45245           
  Misses            ?     4361           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <100%> (?)` | |
| #single | `40.19% <75%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.92% <100%> (ø)` | |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <100%> (ø)` | |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.14% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17547?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/17547?src=pr&el=footer). Last update [c65a0f5...87972a5](https://codecov.io/gh/pandas-dev/pandas/pull/17547?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/17547?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@c65a0f5`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17547/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17547   +/-   ##
=========================================
  Coverage          ?    91.2%           
=========================================
  Files             ?      163           
  Lines             ?    49606           
  Branches          ?        0           
=========================================
  Hits              ?    45245           
  Misses            ?     4361           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <100%> (?)` | |
| #single | `40.19% <75%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.14% <100%> (ø)` | |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.92% <100%> (ø)` | |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17547?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/17547?src=pr&el=footer). Last update [c65a0f5...87972a5](https://codecov.io/gh/pandas-dev/pandas/pull/17547?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback Updated and rebased. Not sure I can make it work, but I found where this should go. closing as stale, if you want to work on this, pls ping.