When using kwargs to construct a CalledProcessError, the repr doesn't show those args, and using kwargs also breaks pickling:

>>> import pickle; from subprocess import CalledProcessError
>>> CalledProcessError(2, 'foo')
CalledProcessError(2, 'foo')
>>> CalledProcessError(2, 'foo').returncode
2
>>> CalledProcessError(2, 'foo').cmd
'foo'
>>> CalledProcessError(returncode=2, cmd='foo')
CalledProcessError()
>>> CalledProcessError(returncode=2, cmd='foo').returncode
2
>>> CalledProcessError(returncode=2, cmd='foo').cmd
'foo'
>>> pickle.loads(pickle.dumps(CalledProcessError(2, 'foo')))
CalledProcessError(2, 'foo')
>>> pickle.loads(pickle.dumps(CalledProcessError(returncode=2, cmd='foo')))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 2 required positional arguments: 'returncode' and 'cmd'
>>>

This is a problem not only with CalledProcessError, but with all custom exceptions with overridden __init__. BaseException.__new__ saves positional arguments as the "args" attribute, but ignores keyword arguments. repr() and pickle use "args".

I tried to fix the issue, the attached PR solves the issue of saving the kwargs and unpickling the exception but I was not able to fix a regression I caused in test_memory_error_in_PyErr_PrintEx.

Reviewing Rémi's page made me realise that a big part of the root cause here is pickle support in exceptions predating the introduction of `__getnewargs__` and `__getnewargs_ex__`.