update imports; remove unused
General cleanup.  flake8 doesn't catch unused cimports.

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=h1) Report
> Merging [#18298](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/148ed63f9287cc55f7a2802da300b717d01cabe6?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/18298/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18298      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49880    49880              
==========================================
- Hits        45592    45583       -9     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (ø)` | :arrow_up: |
| #single | `39.42% <40%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18298?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/18298?src=pr&el=footer). Last update [148ed63...eb632c7](https://codecov.io/gh/pandas-dev/pandas/pull/18298?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/18298?src=pr&el=h1) Report
> Merging [#18298](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/148ed63f9287cc55f7a2802da300b717d01cabe6?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/18298/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18298      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49880    49880              
==========================================
- Hits        45592    45583       -9     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <100%> (ø)` | :arrow_up: |
| #single | `39.42% <40%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18298?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/18298?src=pr&el=footer). Last update [148ed63...eb632c7](https://codecov.io/gh/pandas-dev/pandas/pull/18298?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > General cleanup. flake8 doesn't catch unused cimports.

can we make that happen?
 > can we make that happen?

Not that I'm aware of.  I tried piping the source into flake8 via a sed replacement cimport-->import, but it still chokes when it gets to python syntax errors.

Ultimately I think the effort to build+maintain a flake8 patch to handle this would be more than the effort to just manually check the cimports once in a while. Would this be easier if broken up into smaller pieces?  I'm hesitant to touch the affected files while this is outstanding. thanks!