Update eventing helloworld-python dependencies (#4834)

* upgrade Flask to 2.0.1 and python to 3.9

* use latest Flask version 2.0.3 and active Docker image python:3.9-alpine

code-samples/eventing/helloworld/helloworld-python/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:alpine3.7
+FROM python:3.9-alpine
 
 COPY . /app
 

code-samples/eventing/helloworld/helloworld-python/README.md

@@ -52,15 +52,15 @@ cd knative-docs/code-samples/eventing/helloworld/helloworld-python
 1. Add a `requirements.txt` file containing the following contents:
 
     ```bash
-    Flask==1.1.1
+    Flask==2.0.3
     ```
 
 1. In your project directory, create a file named `Dockerfile` and copy the following code
    block into it. For detailed instructions on dockerizing a Go app, see
    [Deploying Go servers with Docker](https://blog.golang.org/docker).
 
     ```docker
-    FROM python:alpine3.7
+    FROM python:3.9-alpine
 
     COPY . /app
 

code-samples/eventing/helloworld/helloworld-python/requirements.txt

@@ -1 +1 @@
-Flask==1.1.1
+Flask==2.0.3