Define __instancecheck__ on constrained-type metaclasses, to support isinstance() semantics

[Zac-HD]

Hypothesis has a test that for all types that have a registered strategy, the generated examples are instances of that type. However, this is not true for Pydantic's custom types... yet. For example:

t = confloat(ge=0.0, lt=1.0)

assert isinstance(0.5, t)  # currently fails, could pass
assert isinstance(2.5, t)  # would keep failing

The trick is to define the __instancecheck__ magic method, which can be used to customise the behaviour of isinstance() - in this case, by treating standard int, float, str, etc. instances as instances of the constrained types if they happen to match the constraints.