Bump python version to 3.9 to address several vulnerability problems

FYI, I made these changes with the goal of addressing the issues shown in the vulnerability scan of the Docker image because, as per Amol's input, the vulnerabilities were related to libraries used in the python Docker image

Additionally, I removed some of the packages from the requirements file because they are already installed when Flask get installed because they are Flask's dependencies. Moreover, `dataclasses` got integrated into Python in version 3.7

Change-Id: I41fdfbde37cecb59bcdc728692ae33bd40a487e9
diff --git a/docker/Dockerfile b/docker/Dockerfile
index 39e3e97..3f2c182 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -3,7 +3,7 @@
 # SPDX-FileCopyrightText: 2020-present Open Networking Foundation <info@opennetworking.org>
 # SPDX-License-Identifier: Apache-2.0
 #
-FROM python:3.6-slim-buster
+FROM python:3.9-slim-buster
 
 RUN useradd -ms /bin/bash ubuntu
 USER ubuntu
diff --git a/requirements.txt b/requirements.txt
index 559baee..8b5440e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,25 +3,7 @@
 # SPDX-FileCopyrightText: 2020-present Open Networking Foundation <info@opennetworking.org>
 # SPDX-License-Identifier: Apache-2.0
 #
-aniso8601==9.0.1
-certifi==2021.10.8
-charset-normalizer==2.0.11
-click==8.0.3
-dataclasses==0.8
-Flask==2.0.2
-Flask-RESTful==0.3.9
-idna==3.3
-importlib-metadata==4.8.3
-itsdangerous==2.0.1
-Jinja2==3.0.3
-MarkupSafe==2.0.1
-prometheus-client==0.13.1
+Flask==2.0.3
+prometheus_client==0.14.1
 pyaml==21.10.1
-pytz==2021.3
-PyYAML==6.0
-requests==2.27.1
-six==1.16.0
-typing-extensions==4.0.1
-urllib3==1.26.8
-Werkzeug==2.0.2
-zipp==3.6.0
+requests==2.22.0
\ No newline at end of file