Release 0.9.1

Release 0.9.1 is a minor release and contains several bugfixes and some (typing) improvements. This release also introduces on_final callbacks on machines (as well as NestedState) and final flags for states.

• Bug #594: Fix may_ always returning false for internal transitions (thanks @a-schade)
• PR #592: Pass investigated transition to EventData context in 'may' check (thanks @msclock)
• PR #634: Improve the handling of diagrams when working with parallel states, especially when using the show_roi option (thanks @seanxlliu)
• Bug #619/#639: Exception is not broad enough and does not catch asyncio.CancelledError or KeyboardInterrupt; use BaseException instead (thanks @e0lithic and @ofacklam)
• '_anchor' suffix has been removed for (py)graphviz cluster node anchors
• local testing switched from tox to nox
• PR #633: Remove surrounding whitespace from docstrings (thanks @artofhuman)
• PR #665: Add on_final to Machine and NestedState constructor and final to State. Callbacks passed to on_final will be executed when a State with final=True is entered or all children of a parallel state are final.
• Bug #626: Process exceptions with Machine.on_exception in may_ as well (thanks @match1)
• PR #666: Improved handling of removing transitions for GraphMachine and HiearachicalMachine (thanks @drpjm)
• PR #667: Better handling of unset styling in GraphMachine
• Typing:
  • Added --strict mypy flag for transitions type checks (not tests though)
  • Made state, event and machine property mandatory in (Nested)EventData
  • Transition.dest may be None
  • Introduced (Async)CallbackFunc to declutter callback-related signatures
  • Add stub file for extension package for explicite reexport (thanks @DrewDevereux)