Changes

pySim/transport: add abstract get_atr method to LinkBase

The implementations that inherit from the LinkBase class are expected to
implement a get_atr method. This method is mandatory, since it is one of
the most basic functionalities of pySim to display an ATR. The ATR is
also used to distinguish between different card models.

Related: OS#6322
Change-Id: I4fc020ca45658af78e495a5c1b985213f83cbb50