Plugin interfaces: port to Python 3 ABC syntax

And test that the interface (only) classes can not be used directly.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/plugin_interfaces.py

@@ -16,9 +16,7 @@
 import abc
 
 
-class Plugin:
-
-    __metaclass__ = abc.ABCMeta
+class Plugin(metaclass=abc.ABCMeta):
 
     """
     Base for all plugins

selftests/unit/test_plugin_interfaces.py

+import unittest
+
+from avocado.core import plugin_interfaces
+
+
+class Plugin(unittest.TestCase):
+
+    def test_instantiate_settings(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.Settings()
+
+    def test_instantiate_cli(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.CLI()
+
+    def test_instantiate_cli_cmd(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.CLICmd()
+
+    def test_instantiate_job_pre(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.JobPre()
+
+    def test_instantiate_job_post(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.JobPost()
+
+    def test_instantiate_result(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.Result()
+
+    def test_instantiate_job_pre_tests(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.JobPreTests()
+
+    def test_instantiate_job_post_tests(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.JobPostTests()
+
+    def test_instantiate_result_events(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.ResultEvents()
+
+    def test_instantiate_varianter(self):
+        with self.assertRaises(TypeError):
+            # pylint: disable=E0110
+            plugin_interfaces.Varianter()