Always pop `._Cache.resources` AFTER `mng.__aexit__()`

The correct ordering is to de-alloc the surrounding `service_n`
+ `trio.Event` **after** the `mng` teardown ensuring the
`mng.__aexit__()` never can hit a ref-error if it touches either (like
if a `tn` is passed to `maybe_open_context()`!

tractor/trionics/_mngrs.py

@@ -223,16 +223,18 @@ class _Cache:
         task_status: trio.TaskStatus[T] = trio.TASK_STATUS_IGNORED,
 
     ) -> None:
-        async with mng as value:
-            _, no_more_users = cls.resources[ctx_key]
-            cls.values[ctx_key] = value
-            task_status.started(value)
-            try:
-                await no_more_users.wait()
-            finally:
-                # discard nursery ref so it won't be re-used (an error)?
-                value = cls.values.pop(ctx_key)
-                cls.resources.pop(ctx_key)
+        try:
+            async with mng as value:
+                _, no_more_users = cls.resources[ctx_key]
+                try:
+                    cls.values[ctx_key] = value
+                    task_status.started(value)
+                    await no_more_users.wait()
+                finally:
+                    value = cls.values.pop(ctx_key)
+        finally:
+            # discard nursery ref so it won't be re-used (an error)?
+            cls.resources.pop(ctx_key)
 
 
 @acm