Fix render() for Django 1.11

[blag]

According to #28095, the signature of Widget.build_attrs() changed from:

class Widget(...):
    # ...
    def build_attrs(self, attrs, **kwargs):
        # ...

to:

class Widget(...):
    # ...
    def build_attrs(self, attrs, extra_args=None):
        # ...

this fixes DateTimePicker.render() to use the new signature while remaining reverse compatible with Django < 1.11.