chore: add compatibility with the next ops release (#178)
The next release of ops has some backwards-incompatible changes to
private methods. This PR does the minimum to keep Scenario working with
the current version of ops and the next release.
I'll open a ticket for doing a nicer version of this where the new
`_JujuContext` class is used (which would presumably mean requiring the
new version of ops). But this will let people continue upgrading their
ops as long as they're using the latest 6.x of Scenario.
The relevant ops PR is: https://github.com/canonical/operator/pull/1313
