Upgrade databases dependency to >=0.9.0 and SQL Alchemy >= 2.0.0b1

[Iain-S]

Databases 0.9.0 adds support for SQL Alchemy v2, which would allow us to fix this vulnerability. It is not high priority, considering the number of changes it could require and the low risk posed by the vulnerability.

[Iain-S]

Having quickly tried out databases 0.9.0, I think we'd be better off using vanilla SQLAlchemy 2 and not using databases. 0.9.0 caused non-trivial transaction-related errors and/or hanging of our tests. SQLAlchemy 2 supports async natively and is more actively developed.