solo pool now uses apply_target directly

celery/concurrency/solo.py

@@ -9,13 +9,9 @@ from .base import BasePool, apply_target
 class TaskPool(BasePool):
     """Solo task pool (blocking, inline)."""
 
-    def on_start(self):
-        self.pid = os.getpid()
-
-    def on_apply(self, target, args, kwargs, callback=None,
-            accept_callback=None, **_):
-        return apply_target(target, args, kwargs,
-                            callback, accept_callback, self.pid)
+    def __init__(self, *args, **kwargs):
+        super(TaskPool, self).__init__(*args, **kwargs)
+        self.on_apply = apply_target
 
     def _get_info(self):
         return {"max-concurrency": 1,