Add tests for abstract classes.

This prevents Python 3.12 to complain that no test cases were run
and to exit with cod e5 (which breaks maxi_cov).

pyproject.toml

@@ -66,7 +66,6 @@ exclude_lines = [
     "if typing.TYPE_CHECKING:",
     "pragma: no cover",
     "raise AssertionError",
-    "raise NotImplementedError",
     "self.fail\\(\".*\"\\)",
     "@unittest.skip",
 ]

tests/extensions/test_base.py

@@ -1,4 +1,30 @@
+import unittest
+
 from websockets.extensions.base import *
+from websockets.frames import Frame, Opcode
+
+
+class ExtensionTests(unittest.TestCase):
+    def test_encode(self):
+        with self.assertRaises(NotImplementedError):
+            Extension().encode(Frame(Opcode.TEXT, b""))
+
+    def test_decode(self):
+        with self.assertRaises(NotImplementedError):
+            Extension().decode(Frame(Opcode.TEXT, b""))
+
+
+class ClientExtensionFactoryTests(unittest.TestCase):
+    def test_get_request_params(self):
+        with self.assertRaises(NotImplementedError):
+            ClientExtensionFactory().get_request_params()
+
+    def test_process_response_params(self):
+        with self.assertRaises(NotImplementedError):
+            ClientExtensionFactory().process_response_params([], [])
 
 
-# Abstract classes don't provide any behavior to test.
+class ServerExtensionFactoryTests(unittest.TestCase):
+    def test_process_request_params(self):
+        with self.assertRaises(NotImplementedError):
+            ServerExtensionFactory().process_request_params([], [])