Add Python 3.9 to test matrix

.github/workflows/python-tests.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python${{ matrix.python-version }}

tests/wsgi_utils.py

@@ -1,7 +1,7 @@
 import io
 from typing import Dict, Optional, Union, Any, Tuple, List, Callable, Iterable
 
-WSGIEnv = Dict[str, Any]
+WSGIEnv = Dict[str, Any]  # Cannot use TypedDict because of 'HTTP_' variables
 StartResponse = Callable[[str, List[Tuple[str, str]]], None]
 WSGIApp = Callable[[WSGIEnv, StartResponse], Iterable[bytes]]