Hypothesis version 6.165.4

This release improves our heuristic for when to run the "Phase.target"
phase, which optimises scores from "hypothesis.target()". Behaviour is
unchanged for "max_examples" below 1000. For larger budgets we now
start optimising much earlier, and alternate between generation and
optimisation for as long as scores keep improving, instead of running
a single optimisation pass halfway through the run. Exploratory
searches using "target()" with a large budget should now make much
better use of it (issue #3176).

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