Don't rely on sqlalchemy.__all__ being present

The __all__ attribute was removed in v2.0.

src/sqlacodegen/generators.py

@@ -241,7 +241,7 @@ def add_import(self, obj: Any) -> None:
 
             if type_.__name__ in dialect_pkg.__all__:
                 pkgname = dialect_pkgname
-        elif type_.__name__ in sqlalchemy.__all__:  # type: ignore[attr-defined]
+        elif type_.__name__ in dir(sqlalchemy):
             pkgname = "sqlalchemy"
         else:
             pkgname = type_.__module__