Revert  DOC/CI: temp pin matplotlib for doc build #20045
Closes https://github.com/pandas-dev/pandas/issues/20031

Need to check the output of `merging.rst`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20704?src=pr&el=h1) Report
> Merging [#20704](https://codecov.io/gh/pandas-dev/pandas/pull/20704?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5d5a718254f45b4bdc386c360c830df395ec02a?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20704/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20704?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20704      +/-   ##
==========================================
- Coverage   91.84%   91.81%   -0.03%     
==========================================
  Files         153      153              
  Lines       49277    49277              
==========================================
- Hits        45257    45243      -14     
- Misses       4020     4034      +14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.91% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20704?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20704/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20704/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.38% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20704?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/20704?src=pr&el=footer). Last update [d5d5a71...62d48af](https://codecov.io/gh/pandas-dev/pandas/pull/20704?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 https://travis-ci.org/pandas-dev/pandas/jobs/366773025 looks OK. Merging.