DOC/BLD: unpin sphinx to use sphinx 1.7
xref https://github.com/pandas-dev/pandas/issues/18147, closes https://github.com/pandas-dev/pandas/issues/16705

Sphinx 1.7 is released, so let's try out if we can now remove the pinning of sphinx to 1.5.x
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=h1) Report
> Merging [#19687](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/abc4ef9132d8bcc8ad987c18fdd757595fd65d8c?src=pr&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19687/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19687      +/-   ##
==========================================
+ Coverage    91.6%   91.66%   +0.05%     
==========================================
  Files         150      150              
  Lines       48908    48938      +30     
==========================================
+ Hits        44804    44860      +56     
+ Misses       4104     4078      -26
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <ø> (+0.05%)` | :arrow_up: |
| #single | `41.81% <ø> (+0.03%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `87.68% <0%> (-0.3%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.9% <0%> (-0.19%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.02% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.88% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.41% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `95.7% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/apply.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBwbHkucHk=) | `96.74% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/dtypes/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2dlbmVyaWMucHk=) | `100% <0%> (ø)` | :arrow_up: |
| ... and [15 more](https://codecov.io/gh/pandas-dev/pandas/pull/19687/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=footer). Last update [abc4ef9...56770e4](https://codecov.io/gh/pandas-dev/pandas/pull/19687?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 loooks like the build timedout Yep, but it is still using sphinx 1.6.6, so therefore that was to be expected. 
Apparently it's not yet on the defaults channel (it's on conda-forge, but I assume we just use defaults?) i think we also have conda-forge channel on the doc build (or rather u can specify it if u need)
but that’s in the .sh parts Last commit seems to work! No failure, no time-out I think it would be good (in another PR) to update our vendored versions of IPython.sphinxext and numpydoc to their latest versions, so once they are released, it will be easier to remove them. @jorisvandenbossche I think the main channel has been updated to include sphinx 1.7 OK, thanks for noticing. Updated the PR. If it builds, this should be good to go. FYI I won't generally merge doc related PR's (for doc building type things), unless you ping and say ok It was just a general note, not necessarily meant to say that you should merge this :) 
Anyway, it is building! (and not timing out)