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Fix pickle handling for SingletonGate class (#10871)

* Fix pickle handling for SingletonGate class

This commit fixes an oversight in #10314 where the handling of pickle
wasn't done correctly. Because the SingletonGate class defines __new__
and based on the parameters to the gate.__class__() call determines
whether we get a new mutable copy or a shared singleton immutable
instance we need special handling in pickle. By default pickle will call
__new__() without any arguments and then rely on __setstate__ to update
the state in the new object. This works fine if the original instance
was a singleton but in the case of mutable copies this will create a
singleton object instead of a mutable copy. To fix this a __reduce__
method is added to ensure arguments get passed to __new__ forcing a
mutable object to be created in deserialization. Then a __setstate__
method is defined to correctly update the mutable object post creation.

* Use __getnewargs_ex__ insetad of __reduce__ & __setstate__

This commit pivots the pickle interface methods used to implement
__getnewargs_ex__ instead of the combination of __reduce__ and
__setstate__. Realistically, all we need to do here is pass that
we have mutable arguments to new to trigger it to create a separate
object, the rest of pickle was working correctly. This makes the
interface being used for pickle a lot clearer.

* Improve assertion in immutable pickle test

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