fix python2/3 compatibility (#776)

The former code does not allow python 2 unicode strings.
Because of the "OR" the second "isinstance" is evaluated
for python 2 and 3 but should be used for python 3 only.

lib/jnpr/junos/rpcmeta.py

@@ -320,9 +320,8 @@ def _exec_rpc(*vargs, **kvargs):
                     if not isinstance(arg_value, (tuple, list)):
                         arg_value = [arg_value]
                     for a in arg_value:
-                        if ((sys.version < '3' and
-                           not isinstance(a, (bool, str, unicode))) or
-                           not isinstance(a, (bool, str))):
+                        if not isinstance(a, (bool, str, unicode)
+                                          if sys.version < '3' else (bool, str)):
                             raise TypeError("The value %s for argument %s"
                                             " is of %s. Argument "
                                             "values must be a string, "