Support for Widget.render() method without the renderer argument removed in Django 2.1.

[Shivaank121]

There had been this backward-incompatible change in Django 2.1:
https://docs.djangoproject.com/en/2.1/releases/2.1/#features-removed-in-2-1.

Need to fix the method signature of render(), so that it looks like this:

def render(self, name, value, attrs=None, renderer=None):