Configurable validates too often

[jasongrout]

Consider this:

from traitlets.config import Configurable
class A(Configurable):
    x = Int(1)
    
    @validate('x')
    def f(self, proposal):
        print('validate x')
        return proposal.value
A(x=5)

prints:

validate x
validate x

while this:

from traitlets import HasTraits
class A(HasTraits):
    x = Int(1)

    @validate('x')
    def f(self, proposal):
        print('validate x')
        return proposal.value
A(x=5)

only validates x once:

validate x

For some reason, Configurable is validating x twice, but HasTraits is only validating once.

[rmorshea]

This is a special case in the configurable constructor where trait assignments are explicitly done twice in order to override any values that may have been set by a classes' config. This could be resolved by moving config assignment and loading from __init__ to setup_instance which occurs prior to __init__.

[rmorshea]

It's actually easier than that. The second call was required due to the timing of calls to descriptor initialization via instance_init. Originally descriptors were setup in __init__, but now they occur in __new__, thus we can remove the redundant setattr in Configurable.__init__ and simply use super().__init__(**kwargs) instead.

See #391