Revert "Revert "MAINT: Remove Long and WidePanel (#15748)" (#15802)"
With https://github.com/statsmodels/statsmodels/issues/3580 now resolved, we can safely revert #15802
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18341?src=pr&el=h1) Report
> Merging [#18341](https://codecov.io/gh/pandas-dev/pandas/pull/18341?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/27581e919a4e76113e2ee0a6e1c598507ab35f0d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18341/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18341?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18341      +/-   ##
==========================================
+ Coverage   91.82%   91.83%   +<.01%     
==========================================
  Files         153      153              
  Lines       49503    49495       -8     
==========================================
- Hits        45458    45454       -4     
+ Misses       4045     4041       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18341?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/18341/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.87% <ø> (+0.57%)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/18341/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18341?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/18341?src=pr&el=footer). Last update [27581e9...854be64](https://codecov.io/gh/pandas-dev/pandas/pull/18341?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Let's still wait until there is an actual statsmodels release. At this point we don't know yet who of statsmodels or pandas will release first. going to merge #18333 which will causes this to fail. We weren't fully testing things in the downstream. @gfyoung go ahead and rebase. This *should* fail. We can decide whether to xfail the test at a later point. > Let's still wait until there is an actual statsmodels release. At this point we don't know yet who of statsmodels or pandas will release first.

@jorisvandenbossche : That's unfortunate, as they said that it would come out by end of summer.  Not sure what happened there.

> We can decide whether to xfail the test at a later point.

@jreback : Not sure what "the test" is, but yep, will do. Ummm...@jreback : what test was supposed to fail? :smile:  > @jorisvandenbossche : That's unfortunate, as they said that it would come out by end of summer. Not sure what happened there.

Well, that's the reality of volunteer-based open source. It's indeed a bit annoying for us to have to keep it, but also not that costly. 
We can just leave the PR open until the new statsmodels is out I would say. > Well, that's the reality of volunteer-based open source. It's indeed a bit annoying for us to have to keep it, but also not that costly.

Yeah...things happen.  I have no problems waiting. > Let's still wait until there is an actual statsmodels release. At this point we don't know yet who of statsmodels or pandas will release first.

IMO pandas should make this change since it provides incentive for statsmodels to drop a release.  @gfyoung once I merge this, it should fail: https://github.com/pandas-dev/pandas/pull/18362

turns out was skipping the tests :< Then I agree with @bashtage we should xfail this test and merge. but let it fail to prove first. > IMO pandas should make this change since it provides incentive for statsmodels to drop a release.

@bashtage : I actually hadn't thought of that.  Also makes sense.

> Then I agree with @bashtage we should xfail this test and merge. but let it fail to prove first.

Sounds good to me. @gfyoung rebase and let's see @jreback : Can confirm that Travis is failing as a result of your PR.  I will `@xfail` the test once Circle and AppVeyor have finished. @jreback : And adding `@xfail` does fix everything.  All is green again. @TomAugspurger is there a statsmodel 0.9.0 release issue? (where we can reference this) @jreback : Judging from the milestone search <a href="https://github.com/statsmodels/statsmodels/milestone/10">here</a>, <a href="https://github.com/statsmodels/statsmodels/issues/3431">statsmodels/statsmodels#3431</a> might be it?  The issue is a little sparse though. I don't think so.

On Sun, Nov 19, 2017 at 12:48 PM, Jeff Reback <notifications@github.com>
wrote:

> @TomAugspurger <https://github.com/tomaugspurger> is there a statsmodel
> 0.9.0 release issue? (where we can reference this)
>
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 Still -1 on merging this. I would say let's try to convince them with words to do a release instead of by breaking their code. ok

@gfyoung let's remove the xfail and just leave this PR until/when statsmodels can do a release.

 Alright.  I removed the `xfail` for the time being.  It might be awhile on this one, as it looks like a lot of stuff is still left for them to do for `0.9` (see <a href="https://github.com/statsmodels/statsmodels/milestone/10">here</a> to track). SM only has to release before 0.22.  I suspect this won't be out until med Q1 2018.  This should be plenty of time.
 Hello @gfyoung! Thanks for updating the PR.













Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on May 16, 2018 at 11:58 Hours UTC @jreback : All is green here. Thanks! Sure thing.  Hopefully we won't need to revert this revert 😄  The deprecating issue was #10892