Invert default value logic for BusABC._is_shutdown. (#1774)

The destructor of BusABC gives a warning when shutdown() was not
previously called on the object after construction.  However, if the
construction fails (e.g. when the derived bus class constructor raises
an exception), there is no way to call shutdown() on the unfinished
object, and it is not necessary either.

Initialize the _is_shutdown flag to False initially and flip it to
True only when the parent class constructor runs, which usually
happens last in derived classes.  That avoids the shutdown warning for
objects that failed to initialize at all.

can/bus.py

@@ -65,7 +65,8 @@ class BusABC(metaclass=ABCMeta):
     #: Log level for received messages
     RECV_LOGGING_LEVEL = 9
 
-    _is_shutdown: bool = False
+    #: Assume that no cleanup is needed until something was initialized
+    _is_shutdown: bool = True
     _can_protocol: CanProtocol = CanProtocol.CAN_20
 
     @abstractmethod
@@ -97,6 +98,10 @@ def __init__(
         """
         self._periodic_tasks: List[_SelfRemovingCyclicTask] = []
         self.set_filters(can_filters)
+        # Flip the class default value when the constructor finishes.  That
+        # usually means the derived class constructor was also successful,
+        # since it calls this parent constructor last.
+        self._is_shutdown: bool = False
 
     def __str__(self) -> str:
         return self.channel_info