Add __getattr__ to flask-sqlalchemy (#6993)

The SQLAlchemy class exposes dynamically classes of SQLAlchemy. The exact classes depend on used SQLAlchemy version.

stubs/Flask-SQLAlchemy/@tests/stubtest_allowlist.txt

@@ -0,0 +1,2 @@
+# Needed due dynamic attribute generation
+flask_sqlalchemy.SQLAlchemy.__getattr__

stubs/Flask-SQLAlchemy/flask_sqlalchemy/__init__.pyi

@@ -84,5 +84,6 @@ class SQLAlchemy:
     def create_all(self, bind: str = ..., app: Any | None = ...) -> None: ...
     def drop_all(self, bind: str = ..., app: Any | None = ...) -> None: ...
     def reflect(self, bind: str = ..., app: Any | None = ...) -> None: ...
+    def __getattr__(self, name: str) -> Any: ...  # exposes dynamically classes of SQLAlchemy
 
 class FSADeprecationWarning(DeprecationWarning): ...