Python 3.8 typing compatibility

Solves problems with

```Traceback (most recent call last):
  File "webui.py", line 201, in <module>
    webui()
  File "webui.py", line 178, in webui
    create_api(app)
  File "webui.py", line 117, in create_api
    from modules.api.api import Api
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\api.py", line 9, in <module>
    from modules.api.models import *
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\models.py", line 194, in <module>
    class SamplerItem(BaseModel):
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\models.py", line 196, in SamplerItem
    aliases: list[str] = Field(title="Aliases")
TypeError: 'type' object is not subscriptable```

and

```Traceback (most recent call last):
  File "webui.py", line 201, in <module>
    webui()
  File "webui.py", line 178, in webui
    create_api(app)
  File "webui.py", line 117, in create_api
    from modules.api.api import Api
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\api.py", line 9, in <module>
    from modules.api.models import *
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\models.py", line 194, in <module>
    class SamplerItem(BaseModel):
  File "H:\AIart\stable-diffusion\stable-diffusion-webui\modules\api\models.py", line 197, in SamplerItem
    options: dict[str, str] = Field(title="Options")
TypeError: 'type' object is not subscriptable```