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/__init__.py

@@ -68,7 +68,7 @@ class LinkBase(abc.ABC):
         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

pySim/transport/calypso.py

@@ -160,5 +160,5 @@ class CalypsoSimLink(LinkBase):
 
         return b2h(data), b2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "osmocon:%s" % (self._sock_path)

pySim/transport/modem_atcmd.py

@@ -170,5 +170,5 @@ class ModemATCommandLink(LinkBase):
         log.debug('Command response: %s, %s',  data, sw)
         return data, sw
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "modem:%s" % self._device

pySim/transport/pcsc.py

@@ -93,5 +93,5 @@ class PcscSimLink(LinkBase):
         # Return value
         return i2h(data), i2h(sw)
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "PCSC:%u[%s]" % (self._reader_number,  self._reader)

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)