gh-154002: Do not wrap a constructor TypeError in pickle._Unpickler

[fedonman]

_instantiate wrapped a constructor TypeError in a new one, passing the traceback as a
second argument, so the traceback object ended up in args and there was no chaining.

The wrapper is now gone and the original error propagates. Its purpose (743d17e, 1998)
was to name the class when __getinitargs__ returned something bogus, and the message
already carries the class name in that case, so the prefix repeated it:

pure Python: ('in constructor for WrongArgCount: WrongArgCount.__init__() takes 2 positional arguments but 3 were given', <traceback object at 0x762b6376b400>)
pure Python  args types: ['str', 'traceback']  cause=NoneType

The C unpickler never wrapped the error, so this also removes the divergence between the
two implementations. Both now give the same result for a bad argument count and for a
TypeError raised inside the constructor body.

The test asserts the original error reaches the caller unchanged. It fails on the
pure-Python unpickler without this change.

For the record, the traceback-in-args part is not from gh-102799 as the issue says. It
comes from 26d95c3, where raise T, V, tb became raise T(V, tb) rather than
raise T(V).with_traceback(tb). gh-102799 only changed sys.exc_info()[2] to
err.__traceback__.

• Issue: pickle: pure-Python _Unpickler mangles a constructor TypeError #154002

[fedonman]

@zware part of EuroPython sprint.