fix[python]: Fix datatype pickling

[OneRaynyDay]

During the __reduce__ phase of a parametric dtype like pl.Datatime("ns") pickling we get:

(<function copyreg._reconstructor(cls, base, state)>,
 (polars.datatypes.Datetime, object, None),
 {'tu': 'ns', 'tz': None})

,but copyreg's reconstructor calls __new__ which we changed to make sure we can support singleton type objects:

def _reconstructor(cls, base, state):
    if base is object:
        obj = object.__new__(cls)
    else:
        obj = base.__new__(cls, state)
        if base.__init__ != object.__init__:
            base.__init__(obj, state)
    return obj

here, object.__new__(cls) returns the cls itself not an instance. We need to have a custom reconstructor in order to get this to work.

[ritchie46]

Thanks for the fix!