cleanup unused imports, constants
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jbrockmendel : Minor `flake8` error.  Otherwise, should be good to go.

Also, I agree with you.  Dependencies, what can I say? 😄  rebase  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=h1) Report
> Merging [#18119](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bc69dc69b168bae59d0ed6a461f18222b8137fa1?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18119/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18119      +/-   ##
==========================================
+ Coverage   91.25%   91.26%   +<.01%     
==========================================
  Files         163      163              
  Lines       50124    50124              
==========================================
+ Hits        45742    45745       +3     
+ Misses       4382     4379       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.07% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.32% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.46% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18119?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/18119?src=pr&el=footer). Last update [bc69dc6...fe5ab80](https://codecov.io/gh/pandas-dev/pandas/pull/18119?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/18119?src=pr&el=h1) Report
> Merging [#18119](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f17aa265021b084ec2b4ff9d97e75700dd24f680?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18119/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18119      +/-   ##
==========================================
- Coverage   91.28%   91.23%   -0.05%     
==========================================
  Files         163      163              
  Lines       50122    50122              
==========================================
- Hits        45752    45731      -21     
- Misses       4370     4391      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.32% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18119?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/18119?src=pr&el=footer). Last update [f17aa26...0611ad2](https://codecov.io/gh/pandas-dev/pandas/pull/18119?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!