ops 2.18.1 and ops-scenario 7.1.2 fixes for deferring and getting the Juju version in tests

What's Changed

Fixes an issue where deferring an event handler could cause another handler of the same event to fail, and one where reading the Juju version in unit tests would incorrectly result in 0.0.0 rather than the version set.

Fixes

• Ensure that the event snapshot is available when one observer defers and another does not in #1562
• Put the Juju version in the environment as well as the JujuContext in #1559
• Maintain transaction integrity on first run in #1558

Documentation

• Set up intersphinx and add links by @tmihoc in #1546

Full Changelog: 2.18.0...2.18.1