Slowly turning a rectangle in pygame

I have some code:

 mouseX, mouseY = pygame.mouse.get_pos()
 angle = math.degrees(math.atan2(mouseY - self.rect.centery, mouseX - self.rect.centerx))
 rotated = pygame.transform.rotate(self.rect, -angle)
 self.rect = lol.get_rect()

How should I modify this code to slowly turn the rectangle depending on the mouse angle (10deg/s etc.)?

Edit: I want to rotate CLASS object (based on pygame.sprite.Sprite class)

Just don't rotate your image to the target angle, but only a little bit until you reach the target angle.

Here's a very simple example. The arrow will slowly follow the mouse:

import pygame, math, base64
pygame.init()
screen = pygame.display.set_mode((200, 200))

img = 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surf = pygame.image.fromstring(base64.b64decode(img, 'UTF-8'), (32, 32), 'RGBA')

def rot_center(image, angle):
    orig_rect = image.get_rect()
    rot_image = pygame.transform.rotate(image, angle)
    rot_rect = orig_rect.copy()
    rot_rect.center = rot_image.get_rect().center
    rot_image = rot_image.subsurface(rot_rect).copy()
    return rot_image

current_angle = 0

while True:
    if pygame.event.get(pygame.QUIT): break
    pygame.event.get()

    mouseX, mouseY = pygame.mouse.get_pos()
    rect = surf.get_rect(center=(92, 92))
    target_angle = math.degrees(math.atan2(mouseY - rect.centery, mouseX - rect.centerx))

    if current_angle > target_angle:
        current_angle -= 0.01
    if current_angle < target_angle:
        current_angle += 0.01 

    screen.fill((40, 140, 40))
    screen.blit(rot_center(surf, -current_angle), rect)
    pygame.display.update()