Downgrade to support Python 3.9

[fabrice102]

Problem

Following #244, the minimum version required for PyTeal is 3.10.

Unfortunately:

• on macOS 12 (latest version of macOS), the default Python version is 3.8.
• if using HomeBrew on macOS, the default version on Homebrew is 3.9. See https://formulae.brew.sh/formula/python@3.9#default
• on Ubuntu 20.04 (previous LTS version that is still used and that is still recommended for https://github.com/algorand/go-algorand - actually the README even says 18.05), the default Python version is 3.8 - installing a new Python requires using a ppa, which may or may not be ok in many high-security environment.

Solution

If 3.8 / 3.9 is sufficient, why not downgrade to Python 3.8 / 3.9?
If this is not, what about adding to the installation guide (https://pyteal.readthedocs.io/en/stable/installation.html) the requirement for installing Python 3.10 with the steps to do so (at least on macOS).

Dependencies

n/a

Urgency

nice to have

[jasonpaulos]

We can't downgrade now that we've started to use 3.10 features (namely pattern matching), but it's certainly possible that we could offer more guidance about how to install/use the correct Python version.

[michaeldiamant]

@fabrice102 We opened #435 to hopefully clarify the docs - feedback welcomed.

Good point RE: updating installation user guide. The PR takes in the feebdack.