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. python-trio/trio#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.

pytest-trio/pytest_trio/plugin.py

Lines 22 to 34 in e930e6f

	try:
	from hypothesis import register_random
	except ImportError: # pragma: no cover
	pass
	else:
	# On recent versions of Hypothesis, make the Trio scheduler deterministic
	# even though it uses a module-scoped Random instance. This works
	# regardless of whether or not the random_module strategy is used.
	register_random(trio._core._run._r)
	# We also have to enable determinism, which is disabled by default
	# due to a small performance impact - but fine to enable in testing.
	# See https://github.com/python-trio/trio/pull/890/ for details.
	trio._core._run._ALLOW_DETERMINISTIC_SCHEDULING = True