Refs #15667 -- Removed support for Widget.render() methods without the renderer argument.

Per deprecation timeline.

django/forms/boundfield.py

 import datetime
-import warnings
 
 from django.forms.utils import flatatt, pretty_name
 from django.forms.widgets import Textarea, TextInput
-from django.utils.deprecation import RemovedInDjango21Warning
 from django.utils.functional import cached_property
 from django.utils.html import conditional_escape, format_html, html_safe
-from django.utils.inspect import func_accepts_kwargs, func_supports_parameter
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext_lazy as _
 
@@ -102,20 +99,11 @@ class BoundField:
         else:
             name = self.html_initial_name
 
-        kwargs = {}
-        if func_supports_parameter(widget.render, 'renderer') or func_accepts_kwargs(widget.render):
-            kwargs['renderer'] = self.form.renderer
-        else:
-            warnings.warn(
-                'Add the `renderer` argument to the render() method of %s. '
-                'It will be mandatory in Django 2.1.' % widget.__class__,
-                RemovedInDjango21Warning, stacklevel=2,
-            )
         return widget.render(
             name=name,
             value=self.value(),
             attrs=attrs,
-            **kwargs
+            renderer=self.form.renderer,
         )
 
     def as_text(self, attrs=None, **kwargs):

docs/ref/forms/widgets.txt

@@ -274,11 +274,6 @@ foundation for custom widgets.
         ``None``, the renderer from the :setting:`FORM_RENDERER` setting is
         used.
 
-        .. versionchanged:: 1.11
-
-            The ``renderer`` argument was added. Support for subclasses that
-            don't accept it will be removed in Django 2.1.
-
     .. method:: value_from_datadict(data, files, name)
 
         Given a dictionary of data and this widget's name, returns the value

docs/releases/2.1.txt

@@ -258,3 +258,6 @@ how to remove usage of these features.
 * The ``Model._meta.has_auto_field`` attribute is removed.
 
 * Support for regular expression groups with ``iLmsu#`` in ``url()`` is removed.
+
+* Support for ``Widget.render()`` methods without the ``renderer`` argument
+  is removed.

tests/forms_tests/widget_tests/test_render_deprecation.py

-from django import forms
-from django.test import SimpleTestCase
-from django.utils.deprecation import RemovedInDjango21Warning
-
-
-class RenderDeprecationTests(SimpleTestCase):
-    def test_custom_widget_renderer_warning(self):
-        class CustomWidget1(forms.TextInput):
-            def render(self, name, value, attrs=None, renderer=None):
-                return super().render(name, value, attrs, renderer)
-
-        class CustomWidget2(forms.TextInput):
-            def render(self, *args, **kwargs):
-                return super().render(*args, **kwargs)
-
-        class CustomWidget3(forms.TextInput):
-            def render(self, name, value, attrs=None):
-                return super().render(name, value, attrs)
-
-        class MyForm(forms.Form):
-            foo = forms.CharField(widget=CustomWidget1)
-            bar = forms.CharField(widget=CustomWidget2)
-            baz = forms.CharField(widget=CustomWidget3)
-
-        form = MyForm()
-        str(form['foo'])  # No warning.
-        str(form['bar'])  # No warning.
-        msg = (
-            "Add the `renderer` argument to the render() method of <class "
-            "'forms_tests.widget_tests.test_render_deprecation"
-            ".RenderDeprecationTests.test_custom_widget_renderer_warning.<locals>"
-            ".CustomWidget3'>. It will be mandatory in Django 2.1."
-        )
-        with self.assertRaisesMessage(RemovedInDjango21Warning, msg):
-            str(form['baz'])