Remove support for python 3.7

[Federico Caselli]

Support for python 3.7 ended earlier this year, and it's being removed also for sqlalchemy 2.1

[sqla-tester]

Federico Caselli has proposed a fix for this issue in the main branch:

Alembic 1.13 now supports Python 3.8 and above https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4975