Fix SQL_QUERY_MUTATOR=None in Celery worker and downgrade mysql-connector-python

Two fixes:

1. SQL_QUERY_MUTATOR=None: superset.sql_lab captures SQL_QUERY_MUTATOR at
   module import time from current_app.config. In Celery's prefork model,
   import_default_modules() runs in child processes before worker_process_init
   (which provides the app context), so the capture can result in None.
   Fix: add superset_celery_app.py wrapper that explicitly sets
   sql_lab.SQL_QUERY_MUTATOR = flask_app.config["SQL_QUERY_MUTATOR"] after
   create_app() completes but before any worker processes are forked.

2. Downgrade mysql-connector-python from 9.0.0 to 8.4.0: version 9.0.0
   was removed from PyPI; 8.4.0 is the latest stable 8.x release and is
   better aligned with SQLAlchemy 1.4 which Superset 4.0.2 targets.

Author: szachovy

services/superset/Dockerfile

@@ -19,7 +19,7 @@ RUN \
     install \
       --no-cache-dir \
       "redis==4.5.4" \
-      "mysql-connector-python==9.0.0"
+      "mysql-connector-python==8.4.0"
 
 USER root
 

services/superset/entrypoint.sh

@@ -19,8 +19,8 @@ if superset test_db \
   /app/set_database_uri.exp
   /usr/bin/run-server.sh &
 
-  celery \
-    --app superset.tasks.celery_app:app worker \
+  PYTHONPATH="/app${PYTHONPATH:+:${PYTHONPATH}}" celery \
+    --app superset_celery_app:app worker \
     --pool prefork \
     --concurrency 4 \
     -O fair &

services/superset/superset_celery_app.py

@@ -0,0 +1,19 @@
+"""
+Custom Celery app wrapper for Superset.
+
+Guarantees that superset.sql_lab.SQL_QUERY_MUTATOR is set to the correct
+function from the Flask app config before the Celery prefork pool forks
+worker processes.
+
+Background: superset.sql_lab captures SQL_QUERY_MUTATOR at module import
+time via config["SQL_QUERY_MUTATOR"] (where config = current_app.config).
+In some timing scenarios with the prefork pool, this capture can result in
+None, causing 'NoneType' object is not callable during async SQL execution.
+"""
+# Import from the standard Superset celery entrypoint (runs create_app())
+from superset.tasks.celery_app import app, flask_app  # noqa: F401
+
+# Explicitly ensure superset.sql_lab has the correct SQL_QUERY_MUTATOR
+# before any worker processes are forked.
+import superset.sql_lab as _sql_lab  # noqa: E402
+_sql_lab.SQL_QUERY_MUTATOR = flask_app.config["SQL_QUERY_MUTATOR"]