code: def get_user_by_password(self, username, password):
          
          try:
              user = self.get(username=username)
          except User.DoesNotExist:
              return None
          
          if bcrypt.hashpw(password, user.pass_hash) == user.pass_hash:
              return user
          else:
              return None
docs: Given a username and a raw, unhashed password, get the corresponding
          user, retuns None if no match is found.