pdb plugin: move entering pdb into a toplevel function

this prepares pdb at collect time

_pytest/pdb.py

@@ -61,6 +61,12 @@ class PdbInvoke:
             return rep
         if hasattr(rep, "wasxfail"):
             return rep
+        return _enter_pdb(item, call.excinfo, rep)
+
+
+
+
+def _enter_pdb(item, excinfo, rep):
     # we assume that the above execute() suspended capturing
     # XXX we re-use the TerminalReporter's terminalwriter
     # because this seems to avoid some encoding related troubles
@@ -71,7 +77,7 @@ class PdbInvoke:
     rep.toterminal(tw)
     tw.sep(">", "entering PDB")
 
-        tb = _postmortem_traceback(call.excinfo)
+    tb = _postmortem_traceback(excinfo)
     post_mortem(tb)
     rep._pdbshown = True
     return rep