tlv: Use self._compute_tag() method rather than direct self.tag

The TLV_IE.from_tlv() method is part of a base class that is inherited
by more specific classes.  The official way to obtain the tag is the
inherited-class-provided self._compute_tag() method, and *not* a direct
reference to the self.tag member.

This allows for some more obscure TLV parsers, such as the upcoming one
for Proactive Commands in the CAT/OTA context.

Change-Id: I0cd70e31567edc5a0584336efcb5e4282734f6dd

pySim/tlv.py

@@ -236,7 +236,7 @@ class TLV_IE(IE):
             return {}, b''
         (rawtag, remainder) = self.__class__._parse_tag_raw(do)
         if rawtag:
-            if rawtag != self.tag:
+            if rawtag != self._compute_tag():
                 raise ValueError("%s: Encountered tag %s doesn't match our supported tag %s" %
                                  (self, rawtag, self.tag))
             (length, remainder) = self.__class__._parse_len(remainder)