BUG: CI and local failures using fastparquet and pyarrow
### 

- [X] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.



### Issue Description

I have a PR which is continually failing at the same point: https://github.com/pandas-dev/pandas/pull/43552/checks?check_run_id=3589495025

The warnings seem to be because within `doc/source/user_guide/scale.rst` the line

```
ts.to_parquet("timeseries.parquet")
``` 
does not execute, and then following objects are not created and generate errors.

Locally, when trying to execute the code I receive the error message in my environment:

```
ImportError: Unable to find a usable engine; tried using: 'pyarrow', 'fastparquet'.
A suitable version of pyarrow or fastparquet is required for parquet support.
Trying to import the above resulted in these errors:
 - Missing optional dependency 'pyarrow'. pyarrow is required for parquet support. Use pip or conda to install pyarrow.
 - Missing optional dependency 'fastparquet'. fastparquet is required for parquet support. Use pip or conda to install fastparquet.
```

Running `pip list` in my Python3.8 environment on MacOs 10.13.6 I can see that:
```
fastparquet 0.6.3
pyarrow  4.0.1
```
are both installed, so this conflicts with the error warning displayed.

This is puzzling for a couple of reasons:

 - firstly because it errors locally and on online CI checks, but I haven't yet spotted any other PRs suffering.
 - for me, I also see local errors on master (not just on my PR branch).
 - my PR seems completely unrelated the CI failures.

If anyone spots anything untoward, please share the wisdom.

