I am not quite sure what you mean by access, but if you are after creating instances of MyNestedClass this is not a problem in jython.
In this case, since MyNestedClass is a non-static nested class, each instance of it requires a reference to an instance of MyClass. To do this in jython:
import mypackage.MyClass
import mypackage.MyClass.MyNestedClass
outer = mypackage.MyClass()
inner = mypackage.MyClass.MyNestedClass(outer)
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