DOC/CI: restore travis CI doc build environment
xref https://github.com/pandas-dev/pandas/pull/26591#issuecomment-498040997

This is one option (adding back the specific doc build environment yml file), another option would be to actually update the main `environment.yaml` file to be suitable for the doc build. 
At the moment, my preference is to add back the separate yml file because 1) those additional dependencies are not necessarily needed for a dev env setup, and just makes that env even more heavy and 2) we might want to pin to certain versions from time to time for the doc build (eg we should consider pinning sphinx to < 2.0, which is not necessarily needed for all contributors to do as well).

Another thing is that this env could actually use an update (eg python 3.6 -> 3.7)

cc @datapythonista 

> 1) those additional dependencies are not necessarily needed for a dev env setup

Although I suppose it is actually the intent of `environment.yaml` to include all optional dependencies as well (it already includes some dependencies that are only needed for the doc build, such as statsmodels and xarray), so not having pyarrow could be regarded as an oversight.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=h1) Report
> Merging [#26621](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/addc5fcd95064b765d4ee4260304d44822fdee3b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26621/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26621      +/-   ##
==========================================
- Coverage   91.86%   91.85%   -0.01%     
==========================================
  Files         174      174              
  Lines       50722    50722              
==========================================
- Hits        46594    46589       -5     
- Misses       4128     4133       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.39% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.81% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26621?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/26621?src=pr&el=footer). Last update [addc5fc...5a76c70](https://codecov.io/gh/pandas-dev/pandas/pull/26621?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/26621?src=pr&el=h1) Report
> Merging [#26621](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/addc5fcd95064b765d4ee4260304d44822fdee3b?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26621/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26621      +/-   ##
==========================================
- Coverage   91.86%   91.85%   -0.01%     
==========================================
  Files         174      174              
  Lines       50722    50722              
==========================================
- Hits        46594    46589       -5     
- Misses       4128     4133       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.39% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26621/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.81% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26621?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/26621?src=pr&el=footer). Last update [addc5fc...5a76c70](https://codecov.io/gh/pandas-dev/pandas/pull/26621?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche looks fine, merge away when ready I guess I'm missing something. I think I added to `environment.yml` all the dependencies to build the docs when I unified the dependencies for local environments. I thought actually that we were already using those for building the docs in travis.

For me it makes much more sense than when we build the documentation locally, we use the same environment as when it's built in travis, I don't see why we would like to have different / less packages, or different versions. If we do what you suggest, upgrading python to 3.7 only for the docs, but not for local environemnts, or similar things, we'll find that we start working on the docs (let's say fixing the warnings), we get something working locally, and then when we upload the changes the docs can fail. I don't see the advantage.

If `environment.yml` is not synced anymore with the travis docs deps (or I made a mistake and they never were), I'd just add whatever is missing. Personally I don't want to have failing examples in my local docs built, for saving few resources on installing couple more dependencies.

But feel free to merge if you disagree, not a big deal. > I guess I'm missing something.

Well, just look at the log output, and you will see that there is something missing in the environment / some things not working in the doc build. 
For azure I saw errors related to pyarrow and clipboard, those might just need to be added to environment.yml. On travis it was actually not even completing the build (erroring on a missing file). I am not really sure how that is related to the environment, but at least it seems to be fixed here, for some reason.

I am fine with unifying the environments, I won't object that (the environment.yml is already huge anyhow .. so that ship has sailed). But let's try to do that again in a proper way in a new PR. So going to merge this, just to have our doc build working again (we already introduced some new errors in recent PRs, which was not visible any more). Thanks for fixing those. I agree on what you say, and it was my bad for not detecting the failure myself. I was planning to check the new docs built, but azure-pipelines started to fail randomly, and the priority was to restore building master, and I didn't time to look much at the travis changes in the logs.

My point was that the docs should build with `environment.yml`, I think we agree on that, will have a look. And hopefully we can get rid of the warnings, and move the docs build to azure and fail the CI in case of errors, and we'll avoid all this kind of problems, which is not the first time we have. We'll have some people working on it this week, let's see if we succeed.