Transport: mark abstract methods with @abc.abstractmethod

transport.py

@@ -30,15 +30,19 @@ class TransportIOError(Exception):
 class Transport(abc.ABC):
     ''' Abstract transport layer for DebugMux '''
 
+    @abc.abstractmethod
     def connect(self, opts: dict) -> None:
         ''' Establish connection to the target and enter DebugMux mode '''
 
+    @abc.abstractmethod
     def disconnect(self) -> None:
         ''' Escape DebugMux mode and terminate connection with the target '''
 
+    @abc.abstractmethod
     def write(self, data: bytes) -> int:
         ''' Write the given data bytes '''
 
+    @abc.abstractmethod
     def read(self, length: int = 0) -> bytes:
         ''' Read the given number of bytes '''