Select Widget's __deepcopy__ does not copy its choices.

[ericfc]

Select inherits its parent's (Widget's) deepcopy which does not take into consideration its choices field and we get weird behavior like below:

from django import forms 

class Fields(forms.Form):
    field = forms.ChoiceField(choices=(('one', 'one'), ('two', 'two')))

class Foo(Fields):
    def __init__(self, *args, **kwargs):
        super(Foo, self).__init__(*args, **kwargs)
        self.fields['field'].widget.choices[0] = ('foo', 'foo')        

class Bar(Fields):
    pass 

foo = Foo()
bar = Bar()

foo.fields['field'].widget.choices
>>>[('foo', 'foo'), ('two', 'two')]

bar.fields['field'].widget.choices
>>>[('foo', 'foo'), ('two', 'two')]

Where foo and bar have nothing to do with one another.
A more simple example:

import copy
from django import forms

widget = forms.Select()
widget_copy = copy.deepcopy(widget)

widget.attrs is widget_copy.attrs
>>>False
widget.choices is widget_copy.choices
>>>True

Since we did a deepcopy the widget.choices is widget_copy.choices must have returned False yet it returned True.

[ericfc]

The pull request has been made:
​https://github.com/django/django/pull/4965