Add python version to main workflow and upgrade action

[emmyoop]

resolves #6203

Description

1. Upgrade actions/setup-python from v2 to v4.3.0 to bring in all the updates for deprecations
2. Defines Python Version to use when installing dependencies

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[emmyoop]

Using 3.8 here to be consistent with everywhere else we define a python version (build step and release process)

[emmyoop]

This is the recommended syntax by the action

[github-actions]

The backport to 1.2.latest failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-1.2.latest 1.2.latest
# Navigate to the new working tree
cd .worktrees/backport-1.2.latest
# Create a new branch
git switch --create backport-6204-to-1.2.latest
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c3ccbe3357d15e45153c70b62bea626d37fc0d05
# Push it to GitHub
git push --set-upstream origin backport-6204-to-1.2.latest
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.2.latest

Then, create a pull request where the base branch is 1.2.latest and the compare/head branch is backport-6204-to-1.2.latest.

[github-actions]

The backport to 1.1.latest failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-1.1.latest 1.1.latest
# Navigate to the new working tree
cd .worktrees/backport-1.1.latest
# Create a new branch
git switch --create backport-6204-to-1.1.latest
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c3ccbe3357d15e45153c70b62bea626d37fc0d05
# Push it to GitHub
git push --set-upstream origin backport-6204-to-1.1.latest
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.1.latest

Then, create a pull request where the base branch is 1.1.latest and the compare/head branch is backport-6204-to-1.1.latest.