DOC: Fixing more doc warings and wrong .. code-block :: directive (space before colon)
Using `.. code-block ::` with a space before the colon wasn't making the blocks be validated for flake8 issues. Same for `ipython` directive.

Making sure the space is not present, and fixing flake8 errors.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=h1) Report
> Merging [#24650](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dc91f4cb03208889b98dc29c1a1fe46b979e81c7?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/24650/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24650      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52384    52384              
==========================================
+ Hits        48390    48391       +1     
+ Misses       3994     3993       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24650/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24650?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/24650?src=pr&el=footer). Last update [dc91f4c...ce0d4e4](https://codecov.io/gh/pandas-dev/pandas/pull/24650?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/24650?src=pr&el=h1) Report
> Merging [#24650](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dc91f4cb03208889b98dc29c1a1fe46b979e81c7?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/24650/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24650      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52384    52384              
==========================================
+ Hits        48390    48391       +1     
+ Misses       3994     3993       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24650/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24650?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/24650?src=pr&el=footer). Last update [dc91f4c...ce0d4e4](https://codecov.io/gh/pandas-dev/pandas/pull/24650?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @datapythonista  Note for the people doing/reviewing/merging PRs that touch a lot of IPython directive code blocks (eg also the linting clean-up ones, if there will still be coming more), can you please check the travis doc build log ? (there was one failure due to this PR)  
It's a bit annoying not to be able to rely on the CI status for that, but that is the reality for now. w/o automated checks that is not possible to manually do things like this
 It is perfectly possible. I do it all the time (when I judge it appropriate to check, like this PR that explicitly tried to fix a lot of ipython directives). it’s possible but requires more effort which we have traditionally not done
green means it passes 

if you don’t agree then we need checks  > if you don’t agree then we need checks

And that is what we are working on (see the issue @datapythonista just opened)