standardize cimports of numpy as "cnp"
In a few files this collects scattered numpy imports and puts them all in one place.

Removes unnecessary `np.import_ufunc()` calls.

- [ ] 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/19405?src=pr&el=h1) Report
> Merging [#19405](https://codecov.io/gh/pandas-dev/pandas/pull/19405?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8a7aca9eda989243fedae4ef525ddef4511ab241?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19405/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19405?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19405   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48729    48729           
=======================================
  Hits        44649    44649           
  Misses       4080     4080
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19405?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/19405?src=pr&el=footer). Last update [8a7aca9...6865cd7](https://codecov.io/gh/pandas-dev/pandas/pull/19405?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. can consider adding a lint rule to enforce some of this, if possible.