7.0 - API adjustments, Pebble check events, and container exec improvements

[tonyandrewmeyer]

A variety of API changes with the goal of adopting Scenario as the new "ops.testing". A lot of breaking changes, but with the intention that these are rare going forward (a similar cadence with ops).

• Removes Context.clear() and Context.cleanup()
• Removes jsonpatch_delta
• Removes the .copy() and .replace() methods of state classes
• Adds tests for Python 3.12, drops <3.8
• Removes various already-deprecated functionality
• Adds consistency checks for StoredState
• Adds tox test-readme to check the code blocks in the README
• Removes scenario.sequences
• Use ctx.on.event_name(event_arg) to specify the event in run()
• Require keyword arguments for many args in state classes
• Use frozensets for most of the state components
• Adds Scenario classes that mirror the ops status classes
• Unify running action events with other events
• Support Pebble check events
• Get a default network with Network, rather than via a classmethod
• Simplified secrets management
• Use the context object for a context manager
• Simplify creating deferred events
• Add the command prefix functionality from Harness to container execs, add the ability to see what execs have been done and how they were called
• Various renames and privacy changes
• Doc improvements

More detailed notes in each of the individual PRs/commits to the branch.

[tonyandrewmeyer]

I'll rebase to fix the conflicts maybe later today or Monday.

[PietroPasotti]

ready to merge!