Add __spec__ to DummyMod

[gpotter2]

Hello, I am having an issue while calling multiprocessing.Process with latest IPython version on python 3.6. This bug does not happen with 2.7.14, and does not seem to be needed to be backported to 5.X. I don’t know about 6.X

Specs: Python 3.6.0, Windows 10 x64

I'm calling a code looking similar to

p = multiprocessing.Process(target=self.trace3D_notebook)
p.start()

Stack trace

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-5260639d868d> in <module>
----> 1 a[0].trace3D()

Z:\Coding\github\scapy\scapy\layers\inet.py in trace3D(self, join)
   1165         import multiprocessing
   1166         p = multiprocessing.Process(target=self.trace3D_notebook)
-> 1167         p.start()
   1168         if join:
   1169             p.join()

D:\Programms\Python3\lib\multiprocessing\process.py in start(self)
    103                'daemonic processes are not allowed to have children'
    104         _cleanup()
--> 105         self._popen = self._Popen(self)
    106         self._sentinel = self._popen.sentinel
    107         _children.add(self)

D:\Programms\Python3\lib\multiprocessing\context.py in _Popen(process_obj)
    221     @staticmethod
    222     def _Popen(process_obj):
--> 223         return _default_context.get_context().Process._Popen(process_obj)
    224
    225 class DefaultContext(BaseContext):

D:\Programms\Python3\lib\multiprocessing\context.py in _Popen(process_obj)
    320         def _Popen(process_obj):
    321             from .popen_spawn_win32 import Popen
--> 322             return Popen(process_obj)
    323
    324     class SpawnContext(BaseContext):

D:\Programms\Python3\lib\multiprocessing\popen_spawn_win32.py in __init__(self, process_obj)
     31
     32     def __init__(self, process_obj):
---> 33         prep_data = spawn.get_preparation_data(process_obj._name)
     34
     35         # read end of pipe will be "stolen" by the child process

D:\Programms\Python3\lib\multiprocessing\spawn.py in get_preparation_data(name)
    170     # or through direct execution (or to leave it alone entirely)
    171     main_module = sys.modules['__main__']
--> 172     main_mod_name = getattr(main_module.__spec__, "name", None)
    173     if main_mod_name is not None:
    174         d['init_main_from_name'] = main_mod_name

AttributeError: 'DummyMod' object has no attribute '__spec__'

Explanation

Multiprocessing fail as DummyMod does not contain a __spec__ field.
According to Python's source code, the code will handle a None value properly: https://github.com/python/cpython/blob/bd73e72b4a9f019be514954b1d40e64dc3a5e81c/Lib/multiprocessing/spawn.py#L169-L183

This patch fixes the issue on my end.