Remove pandas.io.formats.css from MyPy Blacklist
There was a seemingly unused class variable which was failing mypy due to a lack of annotation. On closer inspection however it doesn't appear that this was even used, so I figured deletion makes things simpler.

I suppose if this was subclassed it could have some effect but this isn't part of the exposed API

@jnothman if you have any thoughts or objections
I suppose it doesn't have much use here, for now at least... Don't mind. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=h1) Report
> Merging [#25900](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/882961df6025b800b899d351c243412e799accf4?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25900/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25900      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         175      175              
  Lines       52808    52807       -1     
==========================================
  Hits        48338    48338              
+ Misses       4470     4469       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.82% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.4% <100%> (ø)` | :arrow_up: |
| [pandas/io/formats/css.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3NzLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.56% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25900?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/25900?src=pr&el=footer). Last update [882961d...80f0523](https://codecov.io/gh/pandas-dev/pandas/pull/25900?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/25900?src=pr&el=h1) Report
> Merging [#25900](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/882961df6025b800b899d351c243412e799accf4?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25900/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25900      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         175      175              
  Lines       52808    52807       -1     
==========================================
  Hits        48338    48338              
+ Misses       4470     4469       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.82% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.4% <100%> (ø)` | :arrow_up: |
| [pandas/io/formats/css.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3NzLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25900/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.56% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25900?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/25900?src=pr&el=footer). Last update [882961d...80f0523](https://codecov.io/gh/pandas-dev/pandas/pull/25900?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @WillAyd 