Don't manually save tracebacks in Python 3

Python 3 automatically saves the traceback with exceptions and uses
it when reraising the exception, so we don't have to manually save
the traceback.  Also, just use "raise" instead of six.reraise
when simply choosing to raise the exception without saving it.

Author: DirectXMan12

gssapi/_utils.py

@@ -64,31 +64,55 @@ def enc(x):
     return dict((enc(k), enc(v)) for k, v in six.iteritems(d))
 
 
+# in case of Python 3, just use exception chaining
 @deco.decorator
 def catch_and_return_token(func, self, *args, **kwargs):
     try:
         return func(self, *args, **kwargs)
     except GSSError as e:
         if e.token is not None and self.__DEFER_STEP_ERRORS__:
             self._last_err = e
-            self._last_tb = sys.exc_info()[2]
+            # skip the "return func" line above in the traceback
+            if six.PY2:
+                self._last_tb = sys.exc_info()[2].tb_next.tb_next
+            else:
+                self._last_err.__traceback__ = e.__traceback__.tb_next
 
             return e.token
         else:
-            six.reraise(*sys.exc_info())
+            raise
 
 
 @deco.decorator
 def check_last_err(func, self, *args, **kwargs):
     if self._last_err is not None:
         try:
-            six.reraise(type(self._last_err), self._last_err, self._last_tb)
+            if six.PY2:
+                six.reraise(type(self._last_err), self._last_err,
+                            self._last_tb)
+            else:
+                # NB(directxman12): not using six.reraise in Python 3 leads
+                #                   to cleaner tracebacks, and raise x is valid
+                #                   syntax in Python 3 (unlike raise x, y, z)
+                raise self._last_err
         finally:
-            del self._last_tb  # in case of cycles, break glass
+            if six.PY2:
+                del self._last_tb  # in case of cycles, break glass
+
             self._last_err = None
     else:
         return func(self, *args, **kwargs)
 
+    @deco.decorator
+    def check_last_err(func, self, *args, **kwargs):
+        if self._last_err is not None:
+            try:
+                raise self._last_err
+            finally:
+                self._last_err = None
+        else:
+            return func(self, *args, **kwargs)
+
 
 class CheckLastError(type):
     def __new__(cls, name, parents, attrs):