How to set email as required, if from a model?

forms.py

from django.contrib.auth.models import User
from django import forms
from django.core.exceptions import ValidationError


class UserForm(forms.ModelForm):
    password = forms.CharField(widget=forms.PasswordInput())

    class Meta:
        model = User
        fields = ('first_name', 'last_name', 'username', 'email', 'password')

    def __init__(self, *args, **kwargs):
        super(forms.ModelForm, self).__init__(*args, **kwargs)
        self.fields['first_name'].widget = forms.TextInput(attrs={
            'class': 'form-control',
            'placeholder': 'First Name'
        })
        self.fields['last_name'].widget = forms.TextInput(attrs={
            'class': 'form-control',
            'placeholder': 'Last Name'
        })

    def clean_email(self):
        email = self.cleaned_data['email']
        if len(email) == 0:
            raise ValidationError("email cannot be empty")
        return email