nat division by timedelta
xref #17876 

Does the whatsnew note still go in 0.20.0.txt?

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=h1) Report
> Merging [#17955](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e1dabf37645f0fcabeed1d845a0ada7b32415606?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17955/graphs/tree.svg?token=eZ4WkYLtcO&height=150&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17955      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50113    50113              
==========================================
- Hits        45723    45714       -9     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (ø)` | :arrow_up: |
| #single | `40.31% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17955?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/17955?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/17955?src=pr&el=footer). Last update [e1dabf3...ac19de6](https://codecov.io/gh/pandas-dev/pandas/pull/17955?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/17955?src=pr&el=h1) Report
> Merging [#17955](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5dd2ea0b3211528ffcfe9b231ce6c00f02918153?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17955/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17955      +/-   ##
==========================================
- Coverage   91.24%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50168    50168              
==========================================
- Hits        45778    45769       -9     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.27% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17955?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/17955?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/17955?src=pr&el=footer). Last update [5dd2ea0...7bb3437](https://codecov.io/gh/pandas-dev/pandas/pull/17955?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 will be for 0.21.1, we haven't pushed the whatsnew yet, so will need to update then. > will be for 0.21.1, we haven't pushed the whatsnew yet, so will need to update then.

OK.  Does that mean I need to something now or just be patient? @jbrockmendel : Just hold on for the time being.  This PR is good to go otherwise I think.  We'll ping you once it's added, or if we don't, ping us about it. @jbrockmendel if you would rebase ok ping on green. thanks