 TST: return pytest MarkDecorator from td.skip_if_no
`skip_if_no` returns a decorator that applies a pytest mark.

pytest marks are decorators and can be applied directly to test functions.

This PR changes `skip_if_no` to return just the pytest mark.

`skip_if_no` can then be used in (pytest.param(... , marks=) situations. eg. pandas/tests/io/test_html.py

could also be used in pandas/tests/io/test_excel.py but not done here to avoid overlapping PRs.

`safe_import` could potentially be renamed `_safe_import` since would not be needed directly in tests.

cc @WillAyd 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=h1) Report
> Merging [#26735](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cf25c5cd3ab1f647f780167df622c74b737fa8f5?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26735/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26735      +/-   ##
==========================================
- Coverage   91.78%   91.77%   -0.01%     
==========================================
  Files         174      174              
  Lines       50703    50701       -2     
==========================================
- Hits        46538    46532       -6     
- Misses       4165     4169       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.37% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.8% <100%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `92.98% <100%> (-0.24%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26735?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/26735?src=pr&el=footer). Last update [cf25c5c...8fe9f69](https://codecov.io/gh/pandas-dev/pandas/pull/26735?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/26735?src=pr&el=h1) Report
> Merging [#26735](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cf25c5cd3ab1f647f780167df622c74b737fa8f5?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26735/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26735      +/-   ##
==========================================
- Coverage   91.78%   91.77%   -0.01%     
==========================================
  Files         174      174              
  Lines       50703    50703              
==========================================
- Hits        46538    46534       -4     
- Misses       4165     4169       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.37% <100%> (ø)` | :arrow_up: |
| #single | `41.81% <100%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.22% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26735/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26735?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/26735?src=pr&el=footer). Last update [cf25c5c...c982e19](https://codecov.io/gh/pandas-dev/pandas/pull/26735?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks, possibly worthile to make an issue to identify places where we can fix this in the code (or just do it if not that hard)