MAINT: Remove non-standard and inconsistently-used imports
of nan and ndarray.

Wrap some docstring lines.

Split off from #17083 

 - [ ] 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/17085?src=pr&el=h1) Report
> Merging [#17085](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f9a552dc12262d1d208f9cdc2c5ffd1731f9c361?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `89.47%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17085/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17085      +/-   ##
==========================================
- Coverage   91.02%   91.01%   -0.02%     
==========================================
  Files         161      161              
  Lines       49396    49394       -2     
==========================================
- Hits        44965    44954      -11     
- Misses       4431     4440       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.78% <89.47%> (-0.01%)` | :arrow_down: |
| #single | `40.28% <42.1%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.94% <100%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.02% <100%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.94% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.65% <83.33%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?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/17085?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/17085?src=pr&el=footer). Last update [f9a552d...4e1570b](https://codecov.io/gh/pandas-dev/pandas/pull/17085?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/17085?src=pr&el=h1) Report
> Merging [#17085](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8e6b09ff3a09de58e82da6dcabbfddba61a743d6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `88.88%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17085/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17085      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.02%     
==========================================
  Files         162      162              
  Lines       49497    49494       -3     
==========================================
- Hits        45051    45039      -12     
- Misses       4446     4455       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <88.88%> (-0.01%)` | :arrow_down: |
| #single | `40.26% <38.88%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.94% <100%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95.03% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.02% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <83.33%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17085?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/17085?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/17085?src=pr&el=footer). Last update [8e6b09f...640be57](https://codecov.io/gh/pandas-dev/pandas/pull/17085?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jbrockmendel : LGTM, with some minor comments. lgtm, ex @gfyoung comments. merge on push / green. 1.0 is just FutureWarning

we might not remove until 2.x

 Removing _INDEX_TYPES looks A-OK. @jbrockmendel something went wrong with merge or rebase (big diff now), and also a conflict I'll try to rebase.  If it doesn't work right away, it'll be easier just to do this on a fresh branch. thanks!