Use Python 3.9 base image instead of 3.9.0

Dockerfile

@@ -1,10 +1,10 @@
-FROM python:3.9.0-buster AS build
+FROM python:3.9-buster AS build
 
 COPY . /src
 WORKDIR /src/python
 RUN python3 setup.py bdist_wheel
 
-FROM python:3.9.0-slim-buster
+FROM python:3.9-slim-buster
 ARG TZ
 ENV TZ=$TZ