Fix inscrutable type bug.

Somehow id(PythonInterpreter) != id(cls) on Travis interpreters.

pex/interpreter.py

@@ -312,7 +312,7 @@ def iter_candidates(cls, paths=None):
 
         def iter_interpreters():
             for candidate in cls._find(cls._paths(paths=paths), error_handler=Retain()):
-                if isinstance(candidate, PythonInterpreter):
+                if isinstance(candidate, cls):
                     yield candidate
                 else:
                     python, exception = candidate