transport: add return type annotation to method __str__

The abstract class LinkBase has no return type annotation on its
__str__ method.

Related: OS#6210
Change-Id: I26d3d2714708dbe957704b60d17ba2afa325b2c4
diff --git a/pySim/transport/__init__.py b/pySim/transport/__init__.py
index 1dd8d18..0b50a4f 100644
--- a/pySim/transport/__init__.py
+++ b/pySim/transport/__init__.py
@@ -68,7 +68,7 @@
         self.proactive_handler = proactive_handler
 
     @abc.abstractmethod
-    def __str__(self):
+    def __str__(self) -> str:
         """Implementation specific method for printing an information to identify the device."""
 
     @abc.abstractmethod