Ensure exceptions are pickleable for multiprocessing test runners

[jacebrowning]

Using the library with a multiprocessing test runner (e.g. Django's manage.py test --parallel) results in the socket exceptions raising a TypeError during deserialization. I propose making the exceptions pickealable by building the custom exception message before constructing each class.

Before:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/test_project/.venv/lib/python3.9/site-packages/django/test/runner.py", line 347, in _run_subsuite
    result = runner.run(subsuite)
  File "/test_project/.venv/lib/python3.9/site-packages/django/test/runner.py", line 294, in run
    test(result)
  File "/usr/local/lib/python3.9/unittest/suite.py", line 84, in __call__
    return self.run(*args, **kwds)
  File "/usr/local/lib/python3.9/unittest/suite.py", line 122, in run
    test(result)

  ...

  File "/test_project/.venv/lib/python3.9/site-packages/django/test/runner.py", line 235, in addError
    self.check_picklable(test, err)
  File "/test_project/.venv/lib/python3.9/site-packages/django/test/runner.py", line 168, in check_picklable
    self._confirm_picklable(err)
  File "/test_project/.venv/lib/python3.9/site-packages/django/test/runner.py", line 142, in _confirm_picklable
    pickle.loads(pickle.dumps(obj))
  File "/test_project/.venv/lib/python3.9/site-packages/tblib/pickling_support.py", line 26, in unpickle_exception
    inst = func(*args)

TypeError: __init__() missing 1 required positional argument: 'host'

After:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/local/lib/python3.9/unittest/case.py", line 592, in run
    self._callTestMethod(testMethod)
  File "/usr/local/lib/python3.9/unittest/case.py", line 550, in _callTestMethod
    method()
  File "/usr/local/lib/python3.9/unittest/mock.py", line 1336, in patched
    return func(*newargs, **newkeywargs)
  File "/test_project/test_module.py", line 123, in test_function
    response = client.post(

  ...

  File "/test_project/.venv/lib/python3.9/site-packages/pytest_socket.py", line 246, in guarded_connect
    raise SocketConnectBlockedError(message)

pytest_socket.SocketConnectBlockedError: A test tried to use socket.socket.connect()
  with host "192.0.80.239" (allowed: "0.0.0.0,127.0.0.1").

[codeclimate]

Code Climate has analyzed commit 345b5c6 and detected 2 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2

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