ENH - Modify Dataframe.select_dtypes to accept scalar values
This commit relates to GH16855. It allows the Dataframe.select_dtypes
function to accept scalar values as well as list-like values. As such,
it should maintain backwards compatibility.

 - [x ] closes #16855
 - [x ] tests added / passed
 - [x ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [x ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=h1) Report
> Merging [#16860](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18f929fa83d2d1f335f8ccf325c05a6ce314b94d?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/16860/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16860      +/-   ##
==========================================
- Coverage   90.98%   90.96%   -0.02%     
==========================================
  Files         161      161              
  Lines       49300    49301       +1     
==========================================
- Hits        44857    44849       -8     
- Misses       4443     4452       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <100%> (ø)` | :arrow_up: |
| #single | `40.17% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16860?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/16860?src=pr&el=footer). Last update [18f929f...2f9ae6e](https://codecov.io/gh/pandas-dev/pandas/pull/16860?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 As discussed in commit message. Implemented all changes apart from the logic change as it misses a necessary edge case. Happy to discuss it though. nice PR!

keep em coming!