move pdb plugin post morten traceback selection to a own function

this is preparation for making it resillent against broken envs
that can't import doctest

_pytest/pdb.py

@@ -70,16 +70,21 @@ class PdbInvoke:
         tw.sep(">", "traceback")
         rep.toterminal(tw)
         tw.sep(">", "entering PDB")
-        # A doctest.UnexpectedException is not useful for post_mortem.
+
-        # Use the underlying exception instead:
+        tb = self._postmortem_traceback(call.excinfo)
-        if isinstance(call.excinfo.value, py.std.doctest.UnexpectedException):
-            tb = call.excinfo.value.exc_info[2]
-        else:
-            tb = call.excinfo._excinfo[2]
         post_mortem(tb)
         rep._pdbshown = True
         return rep
 
+    @staticmethod
+    def _postmortem_traceback(excinfo):
+        # A doctest.UnexpectedException is not useful for post_mortem.
+        # Use the underlying exception instead:
+        if isinstance(excinfo.value, py.std.doctest.UnexpectedException):
+            return excinfo.value.exc_info[2]
+        else:
+            return excinfo._excinfo[2]
+
 def post_mortem(t):
     pdb = py.std.pdb
     class Pdb(pdb.Pdb):