Fix support for Python 3.8

Python 3.8 does not have `typing.Annotated` built-in.
The `typing-extensions` package has corresponding backports.

imagecast/api.py

@@ -2,9 +2,10 @@
 # (c) 2020-2021 Andreas Motl <andreas@terkin.org>
 # License: GNU Affero General Public License, Version 3
 import logging
+import sys
 from dataclasses import dataclass
 from functools import lru_cache
-from typing import List, Annotated
+from typing import List
 from urllib.parse import urlparse
 
 from fastapi import Depends, FastAPI, HTTPException, Query
@@ -17,6 +18,12 @@
 from imagecast.core import process
 
 
+if sys.version_info < (3, 9):
+    from typing_extensions import Annotated
+else:
+    from typing import Annotated
+
+
 # https://fastapi.tiangolo.com/advanced/settings/
 class Settings(BaseSettings):
     allowed_hosts: List[str] = []

setup.py

@@ -55,6 +55,7 @@
       },
       zip_safe=False,
       install_requires=[
+          "typing-extensions<5; python_version<'3.9'",
           "docopt-ng>=0.6,<1",
           "munch>=2.3,<5",
           "Pillow>=8,<11",