Fix tox and GitHub actions settings

[greyli]

• Remove the branch constraint for pull request triggering
• Use Python 3.12 for main tests
• Use ubuntu-20.04 for Python 3.6 since it's been removed in ubuntu-latest
• Remove Python 2.7 since it's not supported by GitHub Actions anymore (python2.7 will be removed from the python-versions on June 19 actions/setup-python#672)
• Add the missing setup-python step
• Merge the pip install commands

[nickjj]

What do you think about using single quotes around the name for all 3 Python 3.12 names? This would match the others.

[nickjj]

Looks good. I double checked this in the docs at: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#filter-pattern-cheat-sheet

When you omit any value here it defaults to '**' which matches all branch and tag names.

[greyli]

What do you think about using single quotes around the name for all 3 Python 3.12 names? This would match the others.

It's OK since these names don't contain dots or whitespaces. Similar to the settings in Flask.

[macnewbold]

This change looks good to me.

[jeffwidman]

Let's ship this, added a trivial inline code comment for clarity, hope you don't mind. If you do feel strongly about that, let me know so that in the future I know to wait for you.