Optionally configure SQLAlchemy Session with future=True

this avoids the need for setting `cascade_backrefs=False` everywhere

https://docs.sqlalchemy.org/en/14/errors.html#error-s9r1

https://docs.sqlalchemy.org/en/14/orm/session_api.html#sqlalchemy.orm.Session.params.future

rattail/config.py

@@ -1626,6 +1626,11 @@ def make_config(files=None, usedb=None, preferdb=None, env=None, winsvc=None, ex
         from rattail.db.config import configure_versioning
         configure_versioning(config)
 
+    # maybe set "future" behavior for SQLAlchemy
+    if config.getbool('rattail.db', 'sqlalchemy_future_mode', usedb=False):
+        from rattail.db import Session
+        Session.configure(future=True)
+
     return config