Add Python 3.9 to Docker-based verification

tools/verify/Dockerfile

@@ -1,6 +1,6 @@
 ARG PYTHON_VERSION
 
-FROM python:${PYTHON_VERSION}
+FROM python:${PYTHON_VERSION}-slim
 
 RUN pip install --upgrade pip
 RUN pip install tox

verify.sh

@@ -13,6 +13,7 @@ if [ -z "$1" ]; then
     validate 3.6.2
     validate 3.7.2
     validate 3.8.0
+    validate 3.9.0
 else
     echo "Validating only $1..."
     validate "$1"