Fix the type annotation in pub.py. (#814)

In particular, Python 3.11 on RHEL-9 doesn't seem to
like the pipe syntax for an optional argument.  But
we have the Optional marking available from typing,
so switch to that instead.

Signed-off-by: Chris Lalancette <[email protected]>

Author: clalancette

ros2topic/ros2topic/verb/pub.py

@@ -140,7 +140,7 @@ def publisher(
     print_nth: int,
     times: int,
     wait_matching_subscriptions: int,
-    max_wait_time: float | None,
+    max_wait_time: Optional[float],
     qos_profile: QoSProfile,
     keep_alive: float,
 ) -> Optional[str]: