DOC: change doc build to python 3.6
see https://github.com/pandas-dev/pandas/pull/16432
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=h1) Report
> Merging [#16545](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d4f80b0035fcd67c552ca384000ea92c438cc0ad?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/16545/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16545   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files         161      161           
  Lines       51071    51071           
=======================================
  Hits        46348    46348           
  Misses       4723     4723
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.59% <ø> (ø)` | :arrow_up: |
| #single | `40.11% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16545?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/16545?src=pr&el=footer). Last update [d4f80b0...11abbd7](https://codecov.io/gh/pandas-dev/pandas/pull/16545?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/16545?src=pr&el=h1) Report
> Merging [#16545](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d4f80b0035fcd67c552ca384000ea92c438cc0ad?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16545/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16545      +/-   ##
==========================================
+ Coverage   90.75%   90.75%   +<.01%     
==========================================
  Files         161      161              
  Lines       51071    51072       +1     
==========================================
+ Hits        46348    46349       +1     
  Misses       4723     4723
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.59% <ø> (ø)` | :arrow_up: |
| #single | `40.11% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.66% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16545?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/16545?src=pr&el=footer). Last update [d4f80b0...655d22d](https://codecov.io/gh/pandas-dev/pandas/pull/16545?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 With these changes, the doctests now pass. 
But, there seems to be a problem with the clipboard ones by going from PyQt4 to 5 (only 4 is used for clipboard I think). So best to install another package for clipboard? @jreback of all the different possibilities of packages to install for clipboard functionality, a preference to install on travis for the docs? Added `xsel` in the latest commit @jorisvandenbossche lgtm.  I should have also brought this up in the other issue, are we OK with expanding our doctests? I think we should, as they make for better examples, and would have caught a couple things last time around. But they do feel a bit fragile (stuff like dtypes, ordering can break them). I am OK with that, and it's what I have been working (very slowly) towards (eg adding the imports in the conftest file to be able to run them, and on the docathon I let a few people corrects examples based on running the doctests). 

As long we don't see it as tests like the actual unit tests, but as testing that the example is correct, I think they are very useful. Yes, this is ready > But they do feel a bit fragile (stuff like dtypes, ordering can break them).

We should maybe make an overview somewhere of the different tricks to use to get the tests passing. And we should also check if we could do some of the directives (eg normalize whitespace) globally, so you don't need to add them in the actual example code