run-tests: drop a workaround for python2

The problem is only on python3, but the awkward handling was because python2
didn't have this exception type.  I've sporadically seen it running in WSL, but
no clue what it means.

tests/run-tests.py

@@ -272,14 +272,11 b' def checkportisavailable(port):'
         with contextlib.closing(socket.socket(family, socket.SOCK_STREAM)) as s:
             s.bind(('localhost', port))
         return True
+    except PermissionError:
+        return False
     except socket.error as exc:
         if WINDOWS and exc.errno == errno.WSAEACCES:
             return False
-        # TODO: make a proper exception handler after dropping py2.  This
-        #       works because socket.error is an alias for OSError on py3,
-        #       which is also the baseclass of PermissionError.
-        elif isinstance(exc, PermissionError):
-            return False
         if exc.errno not in (
             errno.EADDRINUSE,
             errno.EADDRNOTAVAIL,