SIENTIAPDE-1646

Pin sqlalchemy<2.1 to fix psycopg driver break; use tmp_path in retrain

- sqlalchemy 2.1 defaults postgresql:// URLs to psycopg (v3), but sientia_do
  only ships psycopg2-binary, breaking with "No module named psycopg"
- Replace hardcoded 'tmp' mkdtemp mock with pytest tmp_path fixture in
  retrain_model tests to avoid writing to a fake path

requirements.txt

@@ -1,6 +1,8 @@
 temporalio
 psycopg2-binary
-sqlalchemy
+# SQLAlchemy 2.1 makes psycopg (v3) the default driver for postgresql:// URLs; sientia_do
+# builds that URL and ships only psycopg2-binary, so 2.1 fails with "No module named psycopg".
+sqlalchemy<2.1
 asyncua==1.0.6
 redis
 sientia_do>=1.12.2