chore: Remove support for 3.7.

Dependencies of the library are beginning to require 3.8 or 3.9 or
higher, and it will just become more of a problem. For now, we're okay
to sunset just 3.8.

.github/workflows/test.yml

@@ -20,7 +20,7 @@ jobs:
       matrix:
         # Test our minimum version bound, the highest version available,
         # and something in the middle (i.e. what gets run locally).
-        python-version: ["3.7", "3.10", "3.11", "3.12"]
+        python-version: ["3.8", "3.10", "3.11", "3.12"]
         sqlalchemy-version: ["1.3.0", "1.4.0", "2.0.0"]
         postgres-drivername: ["postgresql+psycopg2", "postgresql+psycopg"]
         exclude: