Fixed #22502 -- Fixed microseconds/default/form interaction

Made explicit lack of microsecond handling by built-in datetime form
fields. Used that explicitness to appropriately nix microsecond
values in bound fields. Thanks Claude Paroz for the review.

django/forms/widgets.py

@@ -419,6 +419,7 @@ class Textarea(Widget):
 
 class DateTimeBaseInput(TextInput):
     format_key = ''
+    supports_microseconds = False
 
     def __init__(self, attrs=None, format=None):
         super(DateTimeBaseInput, self).__init__(attrs)
@@ -846,6 +847,7 @@ class SplitDateTimeWidget(MultiWidget):
     """
     A Widget that splits datetime input into two <input type="text"> boxes.
     """
+    supports_microseconds = False
 
     def __init__(self, attrs=None, date_format=None, time_format=None):
         widgets = (DateInput(attrs=attrs, format=date_format),