Throw an Error on Python 3.9 or Lower AND Update README for 2.x Support

[dvonthenen]

What is the current behavior?

When installing on a system using Python version 3.9 or lower, the install mechanism should error out.

There was an attempt to try an do this, but apparently it didn't work out:
https://github.com/deepgram/deepgram-python-sdk/blob/main/setup.py#L9-L10

We should also update the docs so that we 3.9 and older versions can still use 2.x SDK as is. We will support security bug fixes as needed.

Steps to reproduce

Able to install on 3.9 or lower.

Expected behavior

Throw an error on install.

Please tell us about your environment

Other information

NA