DOC: pin sphinx to version 1.5
xref https://github.com/pandas-dev/pandas/pull/16702#issuecomment-308689543
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=h1) Report
> Merging [#16704](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/52eda7ba8780194735e25a43812316aa7a3ee6b3?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/16704/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16704   +/-   ##
=======================================
  Coverage   90.91%   90.91%           
=======================================
  Files         161      161           
  Lines       49286    49286           
=======================================
  Hits        44810    44810           
  Misses       4476     4476
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=footer). Last update [52eda7b...ee704f4](https://codecov.io/gh/pandas-dev/pandas/pull/16704?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This fixed the doc build (build finishes, no huge set of warnings (only the normal ones :-)), so merging. 

But there still seems to be a remaining slowdown of ca 10 min