Python: Add up-/downgrade scripts

Adds up- and downgrade scripts for the support for type parameter
defaults.

In the upgrade direction we do nothing, matching the behaviour of
`getDefault` not having a result for old databases.

In the downgrade direction, we explicitly remove the relevant child (via
the `py_exprs` database relation) for `TypeVar`, `TypeVarTuple`, and
`ParamSpec` parameters.

Author: tausbn