Consider if we should support pyfakefs

[dimaqq]

While reviewing https://code.launchpad.net/~artemstreltsov/lp-signing/+git/lp-signing/+merge/501696/+index?ss=1 I have checked if the (machine charm's) workload can be easily mocked out using https://pypi.org/project/pyfakefs/

Turns out it cannot out of the box, because ops.testing wants access to real file system, e.g. to find the charm's YAML files.

We could get around that by sprinkling fs.pause() / fs.resume() in a few places... but then how can we detect if this dep is installed and more importantly active via user's fixtures?

[benhoyt]

Similarly, I think this is more of a discussion. Can discuss briefly in daily, or in Madrid? If I'm understanding correctly I don't love the idea, because it'd introduce pyfakefs as a (testing) dependency.