text: Title: ValueError: cannot find context for 'fork' 
  Body: ### Description
  When importing aiomultiprocess, I got this error. 
  
  
  ### Details
  from aiomultiprocess import Pool
  
  Traceback (most recent call last):
  
    File "<ipython-input-39-066f50ec84fc>", line 1, in <module>
      from aiomultiprocess import Pool
  
    File "C:\Users\patlo\Anaconda3\lib\site-packages\aiomultiprocess\__init__.py", line 11, in <module>
      from .core import Pool, Process, Worker
  
    File "C:\Users\patlo\Anaconda3\lib\site-packages\aiomultiprocess\core.py", line 35, in <module>
      context = multiprocessing.get_context("fork")
  
    File "C:\Users\patlo\Anaconda3\lib\multiprocessing\context.py", line 238, in get_context
      return super().get_context(method)
  
    File "C:\Users\patlo\Anaconda3\lib\multiprocessing\context.py", line 192, in get_context
      raise ValueError('cannot find context for %r' % method)
  
  ValueError: cannot find context for 'fork'
  
  * OS:
  * Python version:3.6.3
  * aiomultiprocess version:0.4.0
  * Can you repro on master? Yes
  * Can you repro in a clean virtualenv? NO
  