Update project to support python 3.9

[alxmrs]

Now, Apache Beam's Python SDK supports Python 3.9. Thus, we should target this python version.

To this end, we need to update our tox versions, CI, setup.py file, etc.

[trivedikavya]

hey sir can i work on it ...? i think i go through this steps like

1. Update tox.ini
2. Update CI Configuration
3. Update setup.py like this if some require i can fix it

[alxmrs]

I’d talk to @mahrsee1997 or @deepgabani8 to see what the best implementation path is at this point.