Add types to engine

[qthequartermasterman]

In an attempt to help with HypothesisWorks#3074, I have added type annotations to hypothesis/internal/conjecture/engine.py. I have checked these type hints with both mypy and pyright, and verified a few of the trickier-to-statically-determine types by checking their runtime types in the tests.

There are two instances that I'm still confused about.

1. Can Overrun be a valid value for self.interesting_examples?

In all places but this one sample (lines 389 in this PR), all of the keys for self.interesting_examples are ConjectureResult. However, this assignment may possible be Overrun. Indeed, in one test, test_backend_can_shrink_bytes, this does occur. This seems (to me) to be the only place in the tests where this Overrun assignment occurs. If Overrun is a valid value, then this violates the typing in several other places where self.interesting_examples is accessed, but nothing seemed to ever trip up the asserts I had placed in those places (and since removed). If Overrun is indeed valid, then I will need to update the hints for self.interesting_examples, and I'd like to better understand the guarantees that ensure Overrun is never a value in other places (so the type narrowing asserts are legal). Many, but not all, of those references are in shrink_interesting_examples for example.

            if changed:
                self.save_buffer(data.buffer)
                self.interesting_examples[key] = data.as_result()  # type: ignore
                self.__data_cache.pin(data.buffer)
                self.shrunk_examples.discard(key)

2. Potentially uninitialized variable trial_data because of a caught PreviouslyUnseenBehavior exception

In line 842, trial_data is defined inside of a try-except block. In the happy path, this variable is always defined. On the otherhand, unless I am misunderstanding something, if a PreviouslyUnseenBehavior is thrown, that variable will not be defined, meaning trial_data.observer.killed and a few other later instances will reference an uninitialized variable. This never happens in the tests, and I've certainly never experienced it while using hypothesis.

Am I misunderstanding the flow of the code here? Is this try-except superfluous? Or is there a genuine error in how the exception is handled?

                try:
                    trial_data = self.new_conjecture_data(
                        prefix=prefix, max_length=max_length
                    )
                    self.tree.simulate_test_function(trial_data)
                    continue
                except PreviouslyUnseenBehaviour:
                    pass

                # If the simulation entered part of the tree that has been killed,
                # we don't want to run this.
                assert isinstance(trial_data.observer, TreeRecordingObserver)
                if trial_data.observer.killed:
                    continue

                ...

On top of those two confusions I have, mypy also complains about some string literals as keys to a TypedDict (line 247), because it does not consider the concatenation of two literals to be a literal. See the code snippet below. Personally, I feel like this is an error in mypy--pyright does not have this complaint. If we would like mypy to be fully happy, I can either add a # type: ignore, or refactor the code to only use true literals. If we don't care that mypy complains, then no change is needed.

    def _log_phase_statistics(self, phase: Literal["reuse", "generate", "shrink"]) -> Generator[None, None, None]:
        ...
        finally:
            self.statistics[phase + "-phase"] = {  # The key here is the concatenation of two string literals.
                "duration-seconds": time.perf_counter() - start_time,
                "test-cases": list(self.stats_per_test_case),
                "distinct-failures": len(self.interesting_examples),
                "shrinks-successful": self.shrinks,
            }

Doubtless, I've missed or misunderstood some pieces. I am happy to revise and iterate!