TST: make it possible to run tests without moto installed
a couple more
```
pandas/tests/io/conftest.py:import moto
pandas/tests/io/conftest.py:    moto.mock_s3().start()
pandas/tests/io/conftest.py:    moto.mock_s3().stop()
pandas/tests/io/json/test_compression.py:import moto
pandas/tests/io/json/test_compression.py:    with moto.mock_s3():
pandas/tests/io/test_excel.py:import moto
pandas/tests/io/test_excel.py:        with moto.mock_s3():
``` ah, yes, the difference is that this one is in a conftest, and so then even if you run some specific tests (not related to tests that use moto), it still tries to import it because it is in a conftest.py. 

But will fix the others as well. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18719?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b8b108c`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18719/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18719?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18719   +/-   ##
=========================================
  Coverage          ?    91.6%           
=========================================
  Files             ?      153           
  Lines             ?    51306           
  Branches          ?        0           
=========================================
  Hits              ?    46999           
  Misses            ?     4307           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.46% <ø> (?)` | |
| #single | `40.72% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18719?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/18719?src=pr&el=footer). Last update [b8b108c...cdf82ac](https://codecov.io/gh/pandas-dev/pandas/pull/18719?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!