Bump dev version (#6545)

Workflow: Release stage 2 - auto publish, run: 95

Author: github-actions[bot]

cylc/flow/__init__.py

@@ -52,7 +52,7 @@ def environ_init():
 
 environ_init()
 
-__version__ = '8.4.0'
+__version__ = '8.4.1.dev'
 
 
 def iter_entry_points(entry_point_name):