Clarify story around sync activity cancellation

[cretz]

Is your feature request related to a problem? Please describe.

Our tests only capture cancel on sync activities, they don't prove that cancel can bubble out

Describe the solution you'd like

Make sure there is a good sync way to bubble out a cancel

[cretz]

Reopening. We still want to discuss possible alternatives here to sync cancellation. Requiring the user to throw a CancelledError may not be the friendliest. Maybe we want https://docs.python.org/3/c-api/init.html#c.PyThreadState_SetAsyncExc?

[cretz]

After team discussion, we're going to:

• Use https://docs.python.org/3/c-api/init.html#c.PyThreadState_SetAsyncExc by default (Rust FFI at https://docs.rs/pyo3/0.17.3/pyo3/ffi/fn.PyThreadState_SetAsyncExc.html)
• Allow you to disable this in the decorator (e.g. @activity.defn(no_thread_exception=True))
• Allow to you shield code from having this thrown within it (e.g. with activity.no_thread_exception():) that will throw immediately upon completion (of the last one in the stack if nested)