"set session" in a SQLAlchemy session object

"set session" in a SQLAlchemy session object

I'm using SQLAlchemy for a project, and need to be able to specify a
session variable/setting for one specific call for performance reasons:
set session max_heap_table_size = 1024 * 1024 * 64;
I can of course do this in MySQL directly (on the shell), but how do I set
this session variable in a SQLAlchemy session?