text: Title: Create a pydict Faker with value_types ...
  Body: I'm trying to create a `pydict` faker with only string values to populate a JSONField. So fare I've tried the following methods without look: 
  
      extra = factory.Faker('pydict', nb_elements=10, value_types=['str'])
      -> TypeError: pydict() got an unexpected keyword argument 'value_types'
  
      extra = factory.Faker('pydict', nb_elements=10, ['str'])
      -> SyntaxError: positional argument follows keyword argument
  
      factory.Faker('pydict', nb_elements=10, 'str')
      -> SyntaxError: positional argument follows keyword argument
  
      extra = factory.Faker('pydict', 10, True, 'str')
      -> TypeError: __init__() takes from 2 to 3 positional arguments but 5 were given
  
  How can I specify the `*value_types` part of the pydict faker?
     