Use PEP 675 `LiteralString` to refine database function annotations

[DMRobertson]

PEP 675 proposes a new typing.LiteralString type. The idea is that typecheckers should check that a LiteralString is composed only from string literals (i.e. cannot contain user-provided data), to help mitigate e.g. SQL injection attacks. This sounds like a nice-to-have for us, though it's probably not worth picking up until mypy has support for the type (python/mypy#12554).