Python: Promote SQLAlchemy modeling

Due to the split between `src/` and `lib/`, I was not really able to do
the next step without having moved the SQLAlchemy modeling over to be in
`lib/` as well.

docs/codeql/support/reusables/frameworks.rst

@@ -177,6 +177,7 @@ Python built-in support
    psycopg2, Database
    sqlite3, Database
    peewee, Database ORM
+   SQLAlchemy, Database ORM
    cryptography, Cryptography library
    pycryptodome, Cryptography library
    pycryptodomex, Cryptography library