Avoid importing Hypothesis, for reduced startup time

[Zac-HD]

We currently have a chunk of code (below) to ensure that Hypothesis can make the Trio scheduler deterministic. https://github.com/python-trio/trio/issues/2981 proposes moving that into Trio itself; after which we can delete it here and celebrate (slightly) faster import times for anyone who doesn't use Hypothesis.

https://github.com/python-trio/pytest-trio/blob/e930e6f6e8fb4d2a336dfa1313fb95c91c2c1725/pytest_trio/plugin.py#L22-L34