Upgrade minimum Python version to 3.10

[michaeldiamant]

Problem

Though PyTeal minimally supports Python 3.6, on-going effort (#222) requires access to Python 3.8.

Since a version bump is required, we'd like to use the opportunity to move to the newest minor version: 3.10 (released 10/2021).

• Rationale: Newer language facilities simplify maintaining PyTeal.
• Underlying assumption: It's straightforward for PyTeal developers to configure a functioning PyTeal environment by configuring a project-specific Python version.

Solution

We'll need to do the following:

• Provide advance community notice about the upcoming change.
• Update the build to reflect the change in minimum Python version.
• For PyTeal developer convenience, document how to configure a project-specific Python version.

Dependencies

Provide sufficient notice for the community to hear about the pending change.

Urgency

Minimally bumping to Python 3.8 is needed to release #185.

[algochoi]

We should also try to update the black version to 22.3.0 - it is the latest stable version (we pinned it down to a beta version before this) and should reformat our code with it.