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

pySim/transport/serial.py

@@ -237,5 +237,5 @@ class SerialSimLink(LinkBase):
         # Return value
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "serial:%s" % (self._sl.name)