Django's default authentication works on username and password fields. Email authentication backend will authenticate users based on email and password.

from django.contrib.auth import get_user_model

class EmailBackend(object):
    """
    Custom Email Backend to perform authentication via email
    """
    def authenticate(self, username=None, password=None):
        user_model = get_user_model() 
        try:
            user = user_model.objects.get(email=username)
            if user.check_password(password): # check valid password
                return user # return user to be authenticated
        except user_model.DoesNotExist: # no matching user exists 
            return None 

    def get_user(self, user_id):
        user_model = get_user_model() 
        try:
            return user_model.objects.get(pk=user_id)
        except user_model.DoesNotExist:
            return None

Add this authentication backend to the AUTHENTICATION_BACKENDS setting.

# settings.py
AUTHENTICATION_BACKENDS = (
    'my_app.backends.EmailBackend', 
    ... 
    )