Please upgrade SQLAlchemy to v2

[mingfang]

[bikeshedder]

This is blocked by migra and schemainspect which both depend on that old version of SQLAlchemy. Tusker itself doesn't even use any SQLAlchemy specific features. Sadly both dependencies depend on SQLAlchemy even though they use raw SQL to inspect and diff the database.

[mingfang]

FYI I submitted PR to fix schemainspect djrobstep/schemainspect#89