Update to Python 3.9

[jwhitlock]

We're on Python 3.7, which is getting security updates until 2023-06-27. Python 3.9 was released 2020-10-05, is getting bugfix releases, and will be supported until 2025. Python 3.9 is supported by both our current Django 2.2.27 and our planned upgrade to 3.2 (#1586). We should update Python before Django.

References:

• Python release schedule, with maintenance dates
• Django 3.2 release notes: "Django 3.2 supports Python 3.6, 3.7, 3.8, 3.9, and 3.10 (as of 3.2.9)"
• Django 2.2 release notes: "Django 2.2 supports Python 3.5, 3.6, 3.7, 3.8 (as of 2.2.8), and 3.9 (as of 2.2.17)"