Prepare support for SQLAlchemy 2.0

[philpep]

Require SQLAlchemy>=1.4 and follow recommended migration path for future SQLAlchemy 2.0: https://docs.sqlalchemy.org/en/14/changelog/migration_20.html

Turn SQLALCHEMY_WARN_20=1 in tests and set PYTHONWARNINGS to always::DeprecationWarning so we get DeprecationWarning displayed during tests.

Fix some contruction which are no longer compatible with SQLAlchemy 2:

• import declarative_base from sqlalchemy.orm
• Use table.update().where(...) instead of table.update(...)
• Use table.delete().where(...) instead of table.delete(...)
• Drop use of legacy select() with a list argument
• Drop use of legacy case() with a list argument
• Use session.get(Model, pk) instead of session.query(Model).get(pk)

[MeRuslan]

hello, is this going anywhere?

[corentinclichy]

Do you know if you will be able to merged soon ?