BUG: support for "level=" when reset_index() is called with a flat Index
State: closed

 - [x] closes #16263
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Labels:
name: Bug
name: Indexing
description: Related to indexing on series/frames, not to indexes themselves

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=h1) Report
> Merging [#16266](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e5134306bd47db9f6d0f125d2cafd0b8a789e065?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16266/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16266      +/-   ##
==========================================
- Coverage   90.33%    90.3%   -0.04%     
==========================================
  Files         167      167              
  Lines       50907    50908       +1     
==========================================
- Hits        45989    45974      -15     
- Misses       4918     4934      +16
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.09% <100%> (-0.02%)` | :arrow_down: |
| #single | `40.29% <88.88%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL192ZXJzaW9uLnB5) | `44.65% <0%> (+1.9%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16266?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/16266?src=pr&el=footer). Last update [e513430...d27ae91](https://codecov.io/gh/pandas-dev/pandas/pull/16266?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/16266?src=pr&el=h1) Report
> Merging [#16266](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ba60321d00d686a3137ffe26f919d64e84d43aa0?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/16266/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16266      +/-   ##
==========================================
- Coverage   90.32%    90.3%   -0.02%     
==========================================
  Files         167      167              
  Lines       50907    50908       +1     
==========================================
- Hits        45982    45973       -9     
- Misses       4925     4935      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.09% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.29% <88.88%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <100%> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16266?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/16266?src=pr&el=footer). Last update [ba60321...c301473](https://codecov.io/gh/pandas-dev/pandas/pull/16266?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

minor comment, otherwise lgtm.
Thanks!