⬆ Add support for Python 3.9 (#2298)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

tests/test_typing_python39.py

@@ -0,0 +1,24 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from .utils import needs_py39
+
+
+@needs_py39
+def test_typing():
+    types = {
+        list[int]: [1, 2, 3],
+        dict[str, list[int]]: {"a": [1, 2, 3], "b": [4, 5, 6]},
+        set[int]: [1, 2, 3],  # `set` is converted to `list`
+        tuple[int, ...]: [1, 2, 3],  # `tuple` is converted to `list`
+    }
+    for test_type, expect in types.items():
+        app = FastAPI()
+
+        @app.post("/", response_model=test_type)
+        def post_endpoint(input: test_type):
+            return input
+
+        res = TestClient(app).post("/", json=expect)
+        assert res.status_code == 200, res.json()
+        assert res.json() == expect