Bug: exporting data frames to excel using xlsxwriter with option cons…
…tant_memory set to true, most of the cells are empty. Now raises NotImlementedError. #15392

- [x] closes #15392
- [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/22502?src=pr&el=h1) Report
> Merging [#22502](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/55d176d1cd624f8921379c0ceed06fb8e00625b9?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/22502/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22502   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50776    50776           
=======================================
  Hits        46737    46737           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22502?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/22502?src=pr&el=footer). Last update [55d176d...bb9db22](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Updated pull request to reference issue number on test and assert raised error message. Thanks @gfyoung. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@55d176d`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22502/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22502   +/-   ##
=========================================
  Coverage          ?   92.03%           
=========================================
  Files             ?      169           
  Lines             ?    50780           
  Branches          ?        0           
=========================================
  Hits              ?    46737           
  Misses            ?     4043           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.44% <ø> (?)` | |
| #single | `42.22% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22502?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/22502?src=pr&el=footer). Last update [55d176d...5a50ad9](https://codecov.io/gh/pandas-dev/pandas/pull/22502?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 this needs a more generic soln I can take a crack at writing the data row-by-row to fix the issue. It might be a little complex, but i can probably have something in a few days. Closing as stale. Ping if you'd like to pick it back up