Allow non-Function args to retry

[simonbyrne]

I hit this while trying to use a PyObject

[simonbyrne]

@ararslan it should already be tested. Or is there a standard way for testing "works with a callable non-Function"?

[ararslan]

I dunno, just try passing something that isn't a function (like e.g. a type) and see what happen?

[fredrikekre]

Add

!!! compat "Julia 1.2"
    Before Julia 1.2 this signature was restricted to `f::Function`.

before the examples in the docstring?