fix NamedSignal.__getitem__ for new versions of NumPy + unit tests/slycot

Author: murrayrm

control/iosys.py

@@ -59,7 +59,7 @@ def _parse_key(self, key, labels=None):
                 for item in key:        # use for loop to save item for error
                     keylist.append(self._parse_key(item, labels=labels))
                 key = keylist
-            elif isinstance(key, tuple):
+            elif isinstance(key, tuple) and len(key) > 0:
                 keylist = []
                 keylist.append(
                     self._parse_key(item := key[0], labels=self.signal_labels))

control/tests/lti_test.py

@@ -305,6 +305,7 @@ def test_squeeze_exceptions(self, fcn):
             evalfr(sys, [[0.1j, 1j], [1j, 10j]])
 
 
+@slycotonly
 @pytest.mark.parametrize(
     "outdx, inpdx, key",
     [('y[0]', 'u[1]', (0, 1)),