Python – Django UserCreationForm custom fields

I am trying to create form for user registration and add some custom fields. For doing that, I've subclassed UserCretionForm and added fields as shown in django documentation. Then I've created function-based view and template based on this form. Now, I can successfully create user and this user is added to admin panel as expected. Problem is that, I can't add class and style for this form's fields. Widgets are not working except for username field. I'm adding my scripts here for illustrating my problem more accurately:

forms.py

from django import forms
from django.contrib.auth.forms import UserCreationForm, AuthenticationForm
from django.contrib.auth.models import User


class SignUpForm(UserCreationForm):
    first_name = forms.CharField(max_length=32, help_text='First name')
    last_name = forms.CharField(max_length=32, help_text='Last name')
    email = forms.EmailField(max_length=64, help_text='Enter a valid email address')

    class Meta(UserCreationForm.Meta):
        model = User
        # I've tried both of these 'fields' declaration, result is the same
        # fields = ('username', 'first_name', 'last_name', 'email', 'password1', 'password2', )
        fields = UserCreationForm.Meta.fields + ('first_name', 'last_name', 'email',)

        widgets = {
            'username': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'Username'}),
            'first_name': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'First Name'}),
            'last_name': forms.TextInput(attrs={'class': 'form-control', 'placeholder': 'Last Name'}),
            'email': forms.EmailInput(attrs={'class': 'form-control', 'placeholder': 'Email'}),
            'password1': forms.PasswordInput(attrs={'class': 'form-control', 'placeholder': 'Password'}),
            'password2': forms.PasswordInput(attrs={'class': 'form-control', 'placeholder': 'Password Again'}),
        }

views.py

from django.contrib.auth import login, authenticate
from django.contrib.auth.views import LoginView
from django.shortcuts import render, redirect

from .forms import SignUpForm, SignInForm


def signup(request):
    if request.method == 'POST':
        form = SignUpForm(request.POST)
        if form.is_valid():
            form.save()
            username = form.cleaned_data.get('username')
            raw_password = form.cleaned_data.get('password1')
            user = authenticate(username=username, password=raw_password)
            login(request, user)
            return redirect('/')
    else:
        form = SignUpForm()
    return render

Widgets in 'forms.py' are not working except for 'username' field. In other words, in web browser 'username' input is shown with "class='form-control'" and "placeholder='Username'", but other fields don't have class and placeholder attribute as I expected. What can be the reason?