Add "pyfakefs" to testing dependencies

[travishathaway]

Checklist

• I added a descriptive title
• I searched for other feature requests and couldn't find a duplicate (including also the type-feature tag)
• I confirmed that it's not related to another project (see the above section)

What is the idea?

I would like to have "pyfakefs" added to this project's testing dependencies.

Why is this needed

While writing tests, it is often very useful to have a "virtual" file system. pyfakefs enables this with a nice API exposing virtual file systems as a testing fixture.

This PR has an example of how I intend to use this library:

• CEP-6 Channel Notices #11462

What should happen?

pyfakefs should be added to the projects testing dependencies.

Additional Context

No response

[jezdez]

My worries around adding this as a runtime dependency come from not knowing if it's practical to rely on, from http://jmcgeheeiv.github.io/pyfakefs/release/usage.html#modules-not-working-with-pyfakefs:

A list of Python modules that are known to not work correctly with pyfakefs will be collected here: multiprocessing, subprocess, ...

An alternative would be to use fsspec's memory backend and use it to write new tests. The upside is that given its features we can access the local file system if a test requires it (or even wrap consecutive calls in a cache) without additional overhead of maintenance or change of interface.

A memory backend fsspec pytest fixture is defined in its own tests here and used here. It's also available on the defaults channel already.

[travishathaway]

Thanks for the feedback on this. I am going to adjust currently open PRs accordingly.